Addresses issue #2163

In the situation where the SSH key has invalid permissions/owner, the reconnect-loop keeps failing repeatedly yet stays silent about the reasons. A message must be reported from the default exception handler (added). In addition, the situations where the SSH key owner or permissions are wrong must lead to a proper failure (added). Ideally, though, the owner/permissions check must happen before launching the VM, hence this is not a perfect fix.
Merge pull request #4696 from konstantint/patch-1

core: raise errors if bad key perms/owner [GH-2163]