Add additional exception information from validators

[Magnus Persson]

I had some trouble figuring out exactly why my image uploads weren't working properly via the admin interface. Using the python shell and manually stepping thru the validationprocess of isValidImage() I caught some exceptions where some features of PIL were missing (decoders or somesuch). Would have been swell if the exception message instead was brought into the admin interface instead of the very generic "This isn't a valid image".

[Malcolm Tredinnick]

Internal setup problems aren't appropriate to display as end-user-level form errors.

[(none)]

Milestone post-1.0 deleted