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