Widget should clearly show sequence errors

[blakesweeney]

Sometimes users put in incorrect sequences (invalid characters, bad format etc). The front end should provide a clear messages on what the issues are. If given an invalid character it should say which characters are invalid. Given a bad format it should state that the format should be fasta, or just a plain sequence.

[blakesweeney]

Additionally, when a character converted to an N then we should also give a warning to let users know.