rpcserver: Cleanup getheaders handler.

This makes the code for getheaders more consistent with the rest of the
code in terms of making use of existing error functions and using the
same RPC error codes as other handlers.

While here, it also performs the fetching of headers directly instead of
using a function from server which makes it more tightly coupled.

rpcserver.go

@@ -2152,19 +2152,18 @@ func handleGetHashesPerSec(s *rpcServer, cmd interface{}, closeChan <-chan struc
 
 // handleGetHeaders implements the getheaders command.
 //
-// NOTE: This is a btcsuite extension ported from
+// NOTE: This is a btcsuite extension originally ported from
 // github.com/decred/dcrd.
 func handleGetHeaders(s *rpcServer, cmd interface{}, closeChan <-chan struct{}) (interface{}, error) {
 	c := cmd.(*btcjson.GetHeadersCmd)
+
+	// Fetch the requested headers from chain while respecting the provided
+	// block locators and stop hash.
 	blockLocators := make([]*chainhash.Hash, len(c.BlockLocators))
 	for i := range c.BlockLocators {
 		blockLocator, err := chainhash.NewHashFromStr(c.BlockLocators[i])
 		if err != nil {
-			return nil, &btcjson.RPCError{
-				Code: btcjson.ErrRPCInvalidParameter,
-				Message: "Failed to decode block locator: " +
-					err.Error(),
-			}
+			return nil, rpcDecodeHexError(c.BlockLocators[i])
 		}
 		blockLocators[i] = blockLocator
 	}
@@ -2172,32 +2171,34 @@ func handleGetHeaders(s *rpcServer, cmd interface{}, closeChan <-chan struct{})
 	if c.HashStop != "" {
 		err := chainhash.Decode(&hashStop, c.HashStop)
 		if err != nil {
-			return nil, &btcjson.RPCError{
-				Code:    btcjson.ErrRPCInvalidParameter,
-				Message: "Failed to decode hashstop: " + err.Error(),
-			}
+			return nil, rpcDecodeHexError(c.HashStop)
 		}
 	}
 	blockHashes, err := s.server.locateBlocks(blockLocators, &hashStop)
 	if err != nil {
 		return nil, &btcjson.RPCError{
 			Code: btcjson.ErrRPCDatabase,
-			Message: "Failed to fetch hashes of block " +
-				"headers: " + err.Error(),
-		}
-	}
-	blockHeaders, err := fetchHeaders(s.chain, blockHashes)
-	if err != nil {
-		return nil, &btcjson.RPCError{
-			Code: btcjson.ErrRPCDatabase,
-			Message: "Failed to fetch headers of located blocks: " +
+			Message: "Failed to fetch hashes of block headers: " +
 				err.Error(),
 		}
 	}
+	headers := make([]wire.BlockHeader, 0, len(blockHashes))
+	for i := range blockHashes {
+		header, err := s.chain.FetchHeader(&blockHashes[i])
+		if err != nil {
+			return nil, &btcjson.RPCError{
+				Code: btcjson.ErrRPCBlockNotFound,
+				Message: "Failed to fetch header of block: " +
+					err.Error(),
+			}
+		}
+		headers = append(headers, header)
+	}
 
-	hexBlockHeaders := make([]string, len(blockHeaders))
+	// Return the serialized block headers as hex-encoded strings.
+	hexBlockHeaders := make([]string, len(headers))
 	var buf bytes.Buffer
-	for i, h := range blockHeaders {
+	for i, h := range headers {
 		err := h.Serialize(&buf)
 		if err != nil {
 			return nil, internalRPCError(err.Error(),