Improve error message when the executable path in the launcher is empty

* Show error "executable path is empty" instead of "Unknown error"

connector/syncthingprocess.cpp

@@ -203,6 +203,17 @@ QStringList SyncthingProcess::splitArguments(const QString &arguments)
     return result;
 }
 
+/*!
+ * \brief Emits errorOccurred() as if the specified \a error had occurred and sets the specified \a errorString.
+ * \remarks May be called if a precondition for starting the process (that is checked outside of the SyncthingProcess
+ *          class itself) has failed (instead of starting the process).
+ */
+void SyncthingProcess::reportError(QProcess::ProcessError error, const QString &errorString)
+{
+    setErrorString(errorString);
+    emit errorOccurred(error);
+}
+
 /*!
  * \brief Stops the currently running process gracefully. If it has been stopped, starts the specified \a program with the specified \a arguments.
  * \sa See stopSyncthing() for details about \a currentConnection.

connector/syncthingprocess.h

@@ -40,6 +40,7 @@ public:
     static SyncthingProcess *mainInstance();
     static void setMainInstance(SyncthingProcess *mainInstance);
     static QStringList splitArguments(const QString &arguments);
+    void reportError(QProcess::ProcessError error, const QString &errorString);
 #ifdef LIB_SYNCTHING_CONNECTOR_BOOST_PROCESS
     QProcess::ProcessState state() const;
     void start(const QString &program, const QStringList &arguments, QIODevice::OpenMode openMode = QIODevice::ReadOnly);

widgets/misc/syncthinglauncher.cpp

@@ -123,7 +123,7 @@ void SyncthingLauncher::launch(const Settings::Launcher &launcherSettings)
         return;
     }
     if (!launcherSettings.useLibSyncthing && launcherSettings.syncthingPath.isEmpty()) {
-        emit errorOccurred(QProcess::FailedToStart);
+        m_process.reportError(QProcess::FailedToStart, QStringLiteral("executable path is empty"));
         return;
     }
     if (launcherSettings.useLibSyncthing) {