Properly launch Web Browser as detached process and handle errors if ocurred

Summary: This path allow to launch a webbrowser as a detached process. If the launch fails, it will display an error message and close the debug session properly (when connection exists of not).

Reviewers: kfunk, brauch

Reviewed By: brauch

Subscribers: kossebau, brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D9035