Show error reason when commiting is not possible

Sometimes committing to the VCS is not possible, e.g. when the commit message is empty or no files are checked or if the email address is invalid.

While the error message is already available internally in

bool SubmitEditorWidget::canSubmit(QString *whyNot) const;

it is not propagated to the UI. The submit button is disabled but the reason is not clear.

I suggest to add a tooltip or an error label near the submit button to inform the user.

Logging may also be appropriate.

Report originated from https://forum.qt.io/topic/154165/qtcreator-und-git