Improve error reporting on failure to connect to baseline server

(cherry picked from commit 9d75ff6fa8f8844ff6599b68618821cd8c501757)

tests/arthur/baselineserver/src/baselineserver.cpp

@@ -162,6 +162,7 @@ bool BaselineHandler::establishConnection()
 {
     if (!proto.acceptConnection(&plat)) {
         qWarning() << runId << logtime() << "Accepting new connection from" << proto.socket.peerAddress().toString() << "failed." << proto.errorMessage();
+        proto.sendBlock(BaselineProtocol::Abort, proto.errorMessage().toLatin1());  // In case the client can hear us, tell it what's wrong.
         proto.socket.disconnectFromHost();
         return false;
     }

tests/arthur/common/baselineprotocol.cpp

@@ -374,7 +374,7 @@ bool BaselineProtocol::connect(const QString &testCase, bool *dryrun)
 
     Command cmd = UnknownError;
     if (!receiveBlock(&cmd, &block)) {
-        errMsg += QLS("Failed to get response from server.");
+        errMsg.prepend(QLS("Failed to get response from server. "));
         return false;
     }