Bad dates cause exception on Gov form

[dread]

Reproduction

Using the government form, create a new package with name 'test' and date released of '23/5/0210'. The result is a 500 error and 'Server Error' message.

Affects all versions of CKAN.

Why it's happening

The dates module is raising an exception on the invalid date when saving the date, which is not being caught. The exception should have been raised only during the earlier 'validation' step and that would be caught.