Enahncements related with fitler rows

This was begun just to allow search  filter beginning with '*' (ex: *.doc) and get a better error displayed when a filter error was found. However it required more changes that expected and was side-tracked bu the finding of group users errors..

Improvements in zentyal-core:
	+ Allow search filters with a leading '*'
	+ Better error reporting when choosing a bad search filter
	+ External exceptions from _print method are caught correctly in CGIs

Improvements in zentyal-users ( works also without the changes in zentyal-core):
+ More faster custom row id filter for Users model