Debugger: Instruct the user to pick a debugger setting

When neither c++ nor qml debugging is enabled in the run page the
debugger wont start. Print a message to the Application Output pane
describing where the setting is located that enables debugging.

Change-Id: I580139e62c0fb0f4ae518ce818738165b80f8e0c
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/debuggerruncontrol.cpp

@@ -564,6 +564,11 @@ void DebuggerRunTool::start()
                 cppEngine = createPdbEngine();
                 break;
             default:
+                if (!m_runParameters.isQmlDebugging) {
+                    reportFailure(DebuggerPlugin::tr("Unable to create a debugging engine. "
+                        "Please select a Debugger Setting from the Run page of the project mode."));
+                    return;
+                }
                 // Can happen for pure Qml.
                 break;
         }