RPC: show script verification errors in "signrawtransaction" result

If there are any script verification errors, when using "signrawtransaction", they are shown in the RPC result:

```
// ...

Result:
{
  "hex" : "value",           (string) The hex-encoded raw transaction with signature(s)
  "complete" : true|false,   (boolean) If the transaction has a complete set of signatures
  "errors" : [                 (json array of objects) Script verification errors (if there are any)
    {
      "txid" : "hash",           (string) The hash of the referenced, previous transaction
      "vout" : n,                (numeric) The index of the output to spent and used as input
      "scriptSig" : "hex",       (string) The hex-encoded signature script
      "sequence" : n,            (numeric) Script sequence number
      "error" : "text"           (string) Verification or signing error related to the input
    }
    ,...
  ]
}
```

src/rpcrawtransaction.cpp

@@ -1,5 +1,5 @@
 // Copyright (c) 2010 Satoshi Nakamoto
-// Copyright (c) 2009-2014 The Bitcoin Core developers
+// Copyright (c) 2009-2015 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -13,6 +13,7 @@
 #include "net.h"
 #include "rpcserver.h"
 #include "script/script.h"
+#include "script/script_error.h"
 #include "script/sign.h"
 #include "script/standard.h"
 #include "uint256.h"
@@ -491,6 +492,18 @@ Value decodescript(const Array& params, bool fHelp)
     return r;
 }
 
+/** Pushes a JSON object for script verification or signing errors to vErrorsRet. */
+static void TxInErrorToJSON(const CTxIn& txin, Array& vErrorsRet, const std::string& strMessage)
+{
+    Object entry;
+    entry.push_back(Pair("txid", txin.prevout.hash.ToString()));
+    entry.push_back(Pair("vout", (uint64_t)txin.prevout.n));
+    entry.push_back(Pair("scriptSig", HexStr(txin.scriptSig.begin(), txin.scriptSig.end())));
+    entry.push_back(Pair("sequence", (uint64_t)txin.nSequence));
+    entry.push_back(Pair("error", strMessage));
+    vErrorsRet.push_back(entry);
+}
+
 Value signrawtransaction(const Array& params, bool fHelp)
 {
     if (fHelp || params.size() < 1 || params.size() > 4)
@@ -532,8 +545,18 @@ Value signrawtransaction(const Array& params, bool fHelp)
 
             "\nResult:\n"
             "{\n"
-            "  \"hex\": \"value\",   (string) The raw transaction with signature(s) (hex-encoded string)\n"
-            "  \"complete\": true|false       (boolean) if transaction has a complete set of signature\n"
+            "  \"hex\" : \"value\",           (string) The hex-encoded raw transaction with signature(s)\n"
+            "  \"complete\" : true|false,   (boolean) If the transaction has a complete set of signatures\n"
+            "  \"errors\" : [                 (json array of objects) Script verification errors (if there are any)\n"
+            "    {\n"
+            "      \"txid\" : \"hash\",           (string) The hash of the referenced, previous transaction\n"
+            "      \"vout\" : n,                (numeric) The index of the output to spent and used as input\n"
+            "      \"scriptSig\" : \"hex\",       (string) The hex-encoded signature script\n"
+            "      \"sequence\" : n,            (numeric) Script sequence number\n"
+            "      \"error\" : \"text\"           (string) Verification or signing error related to the input\n"
+            "    }\n"
+            "    ,...\n"
+            "  ]\n"
             "}\n"
 
             "\nExamples:\n"
@@ -568,7 +591,6 @@ Value signrawtransaction(const Array& params, bool fHelp)
     // mergedTx will end up with all the signatures; it
     // starts as a clone of the rawtx:
     CMutableTransaction mergedTx(txVariants[0]);
-    bool fComplete = true;
 
     // Fetch previous transactions (inputs):
     CCoinsView viewDummy;
@@ -683,12 +705,15 @@ Value signrawtransaction(const Array& params, bool fHelp)
 
     bool fHashSingle = ((nHashType & ~SIGHASH_ANYONECANPAY) == SIGHASH_SINGLE);
 
+    // Script verification errors
+    Array vErrors;
+
     // Sign what we can:
     for (unsigned int i = 0; i < mergedTx.vin.size(); i++) {
         CTxIn& txin = mergedTx.vin[i];
         const CCoins* coins = view.AccessCoins(txin.prevout.hash);
         if (coins == NULL || !coins->IsAvailable(txin.prevout.n)) {
-            fComplete = false;
+            TxInErrorToJSON(txin, vErrors, "Input not found or already spent");
             continue;
         }
         const CScript& prevPubKey = coins->vout[txin.prevout.n].scriptPubKey;
@@ -702,13 +727,19 @@ Value signrawtransaction(const Array& params, bool fHelp)
         BOOST_FOREACH(const CMutableTransaction& txv, txVariants) {
             txin.scriptSig = CombineSignatures(prevPubKey, mergedTx, i, txin.scriptSig, txv.vin[i].scriptSig);
         }
-        if (!VerifyScript(txin.scriptSig, prevPubKey, STANDARD_SCRIPT_VERIFY_FLAGS, MutableTransactionSignatureChecker(&mergedTx, i)))
-            fComplete = false;
+        ScriptError serror = SCRIPT_ERR_OK;
+        if (!VerifyScript(txin.scriptSig, prevPubKey, STANDARD_SCRIPT_VERIFY_FLAGS, MutableTransactionSignatureChecker(&mergedTx, i), &serror)) {
+            TxInErrorToJSON(txin, vErrors, ScriptErrorString(serror));
+        }
     }
+    bool fComplete = vErrors.empty();
 
     Object result;
     result.push_back(Pair("hex", EncodeHexTx(mergedTx)));
     result.push_back(Pair("complete", fComplete));
+    if (!vErrors.empty()) {
+        result.push_back(Pair("errors", vErrors));
+    }
 
     return result;
 }