VCS: Simplify submit editor accept/close flow

* Replace message box with an error on the output pane.
* Separate logic for accept and close.

Change-Id: Ib9fa201a1a67ee195086f7a22bc6678a9642452a
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/git/gitplugin.cpp

@@ -250,7 +250,7 @@ public:
     void startRebaseFromCommit(const FilePath &workingDirectory, QString commit);
 
     void updateActions(VcsBasePluginPrivate::ActionState) override;
-    bool submitEditorAboutToClose() override;
+    bool activateCommit() override;
     void discardCommit() override { cleanCommitMessageFile(); }
 
     void diffCurrentFile();
@@ -1380,7 +1380,7 @@ IEditor *GitPluginPrivate::openSubmitEditor(const QString &fileName, const Commi
     return editor;
 }
 
-bool GitPluginPrivate::submitEditorAboutToClose()
+bool GitPluginPrivate::activateCommit()
 {
     if (!isCommitEditorOpen())
         return true;