Promote log message to exception level on artifact load failure
Promote log message to exception level on artifact load failure

Before this patch exact place of where the error happened during
artifact load was hard to pinpoint, because it was not shown in the
stacktrace. With this change exception is no longer just converted to
string and printed, but also a stacktrace of exact location of the error
is printed

Change-Id: I1442b6743e77e2aa47a708ae1c38d7c4a54b8cd1
Closes-Bug: #1543105

rajalokan/glance,stevelle/glance,dims/glance,klmitch/glance,openstack/glance,klmitch/glance,rajalokan/glance,openstack/glance,dims/glance,openstack/glance,stevelle/glance