plugins/git: Set job error before emitResult()
Summary: GitPlugin::copy()->exec() did always return true, even on failure.
Test Plan: Copy-paste files/folders inside a Git root without write permissions on the destination folder is expected to fail. ProjectManagerViewPlugin::pasteFromContextMenu() will display a warning dialog in this case.
Reviewers: mwolff, kfunk
Reviewed By: kfunk
Subscribers: kdevelop-devel
Differential Revision: https://phabricator.kde.org/D4771