Output more helpful error message on bad mpk format

This has tripped up every people on github. The only place to get
the correct master public key is from Electrum itself.

electrumpersonalserver/deterministicwallet.py

@@ -105,7 +105,11 @@ def rewind_one(self, change):
 class SingleSigWallet(DeterministicWallet):
     def __init__(self, mpk):
         super(SingleSigWallet, self).__init__()
-        self.branches = (btc.bip32_ckd(mpk, 0), btc.bip32_ckd(mpk, 1))
+        try:
+            self.branches = (btc.bip32_ckd(mpk, 0), btc.bip32_ckd(mpk, 1))
+        except Exception:
+            raise ValueError("Bad master public key format. Get it from " +
+                "Electrum menu `Wallet` -> `Information`")
         #m/change/i
 
     def pubkey_to_scriptpubkey(self, pubkey):
@@ -163,8 +167,12 @@ class MultisigWallet(DeterministicWallet):
     def __init__(self, m, mpk_list):
         super(MultisigWallet, self).__init__()
         self.m = m
-        self.pubkey_branches = [(btc.bip32_ckd(mpk, 0), btc.bip32_ckd(mpk, 1))
-            for mpk in mpk_list]
+        try:
+            self.pubkey_branches = [(btc.bip32_ckd(mpk, 0), btc.bip32_ckd(mpk,
+                1)) for mpk in mpk_list]
+        except Exception:
+            raise ValueError("Bad master public key format. Get it from " +
+                "Electrum menu `Wallet` -> `Information`")
         #derivation path for pubkeys is m/change/index
 
     def redeem_script_to_scriptpubkey(self, redeem_script):