formatter: improved error reporting
formatter: improved error reporting

* Improves error reporting for bibformat elements to include
  traceback in template when in debug mode, as well as using
  the Flask application logger to report the error.

Signed-off-by: Lars Holm Nielsen <7eaffca9f7cd98839b5821d0d23e09ff764b82cd@cern.ch>

inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio