Fixes #6072 - Report issues on repo syncing and don't lock repository on failure

Repo synchronization is an idempotent operation. There are also situations,
such as wrong url, that might lead to unexpected situations. Therefore,
instead of keeping the task paused, locking other operations to be performed,
just skip the syncing and report the errors.

The bastion task details page now also includes a link to "More details" that
leads to foreman_tasks/show page with even more details information useful for
debugging the issue and filing bugs.