better error message for bad search strings

[brettatoms]

Should provide more information on bad searches. For example the search string, "species where accession.plants.code = 2009" give the following error:
** Error: type object 'Species' has no attribute 'key'

and the following traceback on the console:

view.py(810): Traceback (most recent call last):
  File "./bauble/view.py", line 806, in search
    results.add(strategy.search(text, self.session))
  File "./bauble/view.py", line 527, in search
    results.add(self._get_results_from_query(tokens, session))
  File "./bauble/view.py", line 428, in _get_results_from_query
    resolved = self._resolve_identifiers(mapping, idents)
  File "./bauble/view.py", line 385, in _resolve_identifiers
    props.append(get_prop(parent, identifiers[0]))
  File "./bauble/view.py", line 365, in get_prop
    parent_name = parent.key
AttributeError: type object 'Species' has no attribute 'key'