MINOR: when putting in the wrong email format, there is no warning.

[eddyhuang97]

Reproduce the issue:

1. Go to sign up page
2. Type in a string with wrong email format, like 'test#test.com'
3. There is no indication that the format is incorrect. When user click create account, nothing happens.

A warning stating what the problem is might be helpful

Rationale: Users might be confused about the situation, especially they might not know whether the password do not match or the username is used or incorrect.