core: Log error correctly when connecting domain for removal

When connecting to a storage domain as a part of its removal (or format
in case of ISO domains) if the connection fails, the error is not logged
correctly and instead we get a generic error, this was fixed and instead
the generic error we will get the real cause

Change-Id: Idd7435336acbec41edbdd70568f1dfdce664f875
Bug-Url: https://bugzilla.redhat.com/1003266
Signed-off-by: Tal Nisan <tnisan@redhat.com>