Improve message format when invalid constraint involves multiple fields

[bboyle]

We currently display this:

• Your name: Must be completed
• Your email: Must be an email addresses
• Birth certificate: Your attachments are too large
• Driver licence: Your attachments are too large

When we need something more like this:

• Your name: Must be completed
• Your email: Must be an email addresses
• Your attachments are too large: birth certificate, driver licence

Maybe it should say:

• Your attachments are too large (help)
• And help pops up a lightbox to display the file sizes and offer help?