show users all requirements for signups in case of an error

I was bored.

I added the requirements for the signup data - except for the field email, as the hint: "email has to be a valid email" seems silly to me in that case.

This just adds the requirements to the errors occured. Another possibility would be that we do not show the failed fields in case of an error and show just all requirements. What do you think about it?

This depends on https://github.com/isaacs/npm-www/pull/335 which has to get merged before.