rest: Improve tests and documention of /headers and /block

Summary: This is a backport of Core [[bitcoin/bitcoin#15177 | PR15177]]

Test Plan: `ninja && test/functional/test_runner.py interface_rest`

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8355

doc/REST-interface.md

@@ -27,6 +27,7 @@ To query for a confirmed transaction, enable the transaction index via "txindex=
 `GET /rest/block/notxdetails/<BLOCK-HASH>.<bin|hex|json>`
 
 Given a block hash: returns a block, in binary, hex-encoded binary or JSON formats.
+Responds with 404 if the block doesn't exist.
 
 The HTTP request and response are both handled entirely in-memory, thus making maximum memory usage at least 2.66MB (1 MB max block, plus hex encoding) per request.
 
@@ -36,6 +37,7 @@ With the /notxdetails/ option JSON response will only contain the transaction ha
 `GET /rest/headers/<COUNT>/<BLOCK-HASH>.<bin|hex|json>`
 
 Given a block hash: returns <COUNT> amount of blockheaders in upward direction.
+Returns empty if the block doesn't exist or it isn't in the active chain.
 
 #### Blockhash by height
 `GET /rest/blockhashbyheight/<HEIGHT>.<bin|hex|json>`

test/functional/interface_rest.py

@@ -234,6 +234,22 @@ def run_test(self):
         self.log.info("Test the /block, /blockhashbyheight and /headers URIs")
         bb_hash = self.nodes[0].getbestblockhash()
 
+        # Check result if block does not exists
+        assert_equal(self.test_rest_request(
+            '/headers/1/0000000000000000000000000000000000000000000000000000000000000000'), [])
+        self.test_rest_request(
+            '/block/0000000000000000000000000000000000000000000000000000000000000000',
+            status=404,
+            ret_type=RetType.OBJ)
+
+        # Check result if block is not in the active chain
+        self.nodes[0].invalidateblock(bb_hash)
+        assert_equal(
+            self.test_rest_request(
+                '/headers/1/{}'.format(bb_hash)), [])
+        self.test_rest_request('/block/{}'.format(bb_hash))
+        self.nodes[0].reconsiderblock(bb_hash)
+
         # Check binary format
         response = self.test_rest_request(
             "/block/{}".format(bb_hash), req_type=ReqType.BIN, ret_type=RetType.OBJ)