cli: improve bitcoin-cli error when not connected

Adds a string suggestion `bitcoin-cli -help` as an additional source of
information.

Author: willcl-ark

src/bitcoin-cli.cpp

@@ -827,7 +827,10 @@ static UniValue CallRPC(BaseRequestHandler* rh, const std::string& strMethod, co
         if (response.error != -1) {
             responseErrorMessage = strprintf(" (error code %d - \"%s\")", response.error, http_errorstring(response.error));
         }
-        throw CConnectionFailed(strprintf("Could not connect to the server %s:%d%s\n\nMake sure the bitcoind server is running and that you are connecting to the correct RPC port.", host, port, responseErrorMessage));
+        throw CConnectionFailed(strprintf("Could not connect to the server %s:%d%s\n\n"
+                    "Make sure the bitcoind server is running and that you are connecting to the correct RPC port.\n"
+                    "Use \"bitcoin-cli -help\" for more info.",
+                    host, port, responseErrorMessage));
     } else if (response.status == HTTP_UNAUTHORIZED) {
         if (failedToGetAuthCookie) {
             throw std::runtime_error(strprintf(