Enable the commit form after git commit failure (plus show an error message).
requested to merge verner/kdevelop:fix/BUG-443475-fix-git-commit-errors-disabling-commit-dialog into release/21.12
This fixes a bug 443475 which would lead to the commit form being disabled and unusable when a commit failed for some reason.
Also, it initially disables the form while no project is selected and shows error messages when a git commit fails.