Make username error more informative

[danirabbit]

Instead of just saying that the username is "invalid", use the text that used to be a tooltip.

Since the username is automatically generated when you type in your real name, chances are this field will rarely be empty so the tooltip will probably never be seen.

[jeremypw]

The code looks OK but is there a way to test it without doing an install?

[danirabbit]

@jeremypw This is initial setup, not installer ;p It can be run from Terminal without fear of danger.

Yes, once I get the design and validation nailed on this panel I would like to backport it to Switchboard so that they feel the same