fixed issue with fieldinformation caching

- the mechanism here is to look up information for a field in a set of "tables_by_priority"; if the field is not found in the first table, then the next table would be checked.  
- The last check-in introduced a regression whereby the first failed lookup would fail-fast, instead of progressing to the next in the "tables_by_priority"
- raising the "DoesNotExist" exception signals to the calling function to continue looking for the field.
- eliminated some superfluous logging.

@jmuhlich @gberriz @dwrobel1  
So, the exception here signals to the calling function to keep looking for the field in the next table in the list; (line 83): 
            except (ObjectDoesNotExist,MultipleObjectsReturned) as e:
                if( i+1 == len(tables_by_priority)): 
                    raise Exception(str((type(e), field_or_alias,tables_by_priority, e.args)))
 so I could return null, but then have to change the logic in the calling function as well - perhaps you are suggesting that would be better?