WebSearch: string representation of SEQP exception
WebSearch: string representation of SEQP exception

* Add string representation to InvenioWebSearchMismatchedParensError
  exception, fixing some error registering issues.

inveniosoftware/invenio-records,tiborsimko/invenio-search,inveniosoftware/invenio-records,tiborsimko/invenio-search,inveniosoftware/invenio-search,tiborsimko/invenio-records,tiborsimko/invenio-records,inveniosoftware/invenio-search