844389 - unsuccessful repo deletion rollbacking

If pulp was is unavailable during repository deletion a process failed
without rollbacking changes in candlepin. Now we delete content in
candlepin in such case so there is no inconsistency and you can try to
add repository again.

Also we display a better error notification to user instructing him to
try again. All backtrace is however stored in notification.

One minor fix regarding elastic search indexed models. Some strings can
be nil so we must check their presence, not their length.
Merge pull request #1537 from ares/bug/844389-repository_deletion_and_creation

844389 - unsuccessful repo deletion rollbacking