fix RPC error replies

After pull #4288, RPC messages indicating errors have a Content-Length unrelated
to their actual contents, rendering bitcoin-cli and curl unable to decode the
reply.

This patch sets the Content-Length field based on the actual content returned.

Additionally, pull #4288 clobbered the error descriptions provided in
ErrorReply, which bitcoin-cli relies upon; this patch moves #4288 http-error
descriptions to an HTTPError method, allowing HTTPReply to pass content on
unchanged.

src/rpcprotocol.h

@@ -141,6 +141,8 @@ private:
 };
 
 std::string HTTPPost(const std::string& strMsg, const std::map<std::string,std::string>& mapRequestHeaders);
+std::string HTTPError(int nStatus, bool keepalive,
+                      bool headerOnly = false);
 std::string HTTPReply(int nStatus, const std::string& strMsg, bool keepalive,
                       bool headerOnly = false,
                       const char *contentType = "application/json");