core: Remove usage of ExceptionUtils.getMessage

Removes usage of ExceptionUtils.getMessage, because using
Exception.getMessage along with logging stacktrace is enough.

Change-Id: I113d01c167397456c0cc7b12a112971e482cf56d
Bug-Url: https://bugzilla.redhat.com/1109871
Signed-off-by: Martin Perina <mperina@redhat.com>