Prior to this commit it was possible for Host#online? to raise a
connection error when checking an already offline host. This could
happen because certain methods used for checking if a host is available
call the "connection" method, without catching any errors. As a result,
if this method raises then #online? would also raise. All of this could
lead to GitLab becoming unavailable depending on how many times it would
try to use this offline host.

To work around this we catch the errors that might be raised in
Host#online?. Unlike the code in LoadBalancing::LoadBalancer we don't
need to catch individual PG errors as we don't have to worry about
application errors. This means we only have to catch up to three errors:

1. ActionView::Template::Error (used inside views).

2. ActiveRecord::StatementInvalid (used basically everywhere outside of
   views).

3. PG::Error: used in some rare cases where Rails (for reasons unknown)
   doesn't wrap the original error.