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.