Improve error message for invalid user name on signup

[fkling]

(I'm not sure whether to file this as a feature request or bug report, or whether this is the correct team to assign this too)

• Sourcegraph version: 97031_2021-05-25_5802320

Steps to reproduce:

1. Go to https://sourcegraph.com/sign-up
2. Provide an invalid user name

Expected behavior:

I'm told with characters are valid for the user name.

Actual behavior:

I'm getting the error "Please match the requested format." without knowing what the format is supposed to be.

[github-actions[bot]]

Heads up @alicjasuska @umpox @valerybugakov @5h1ru @pdubroy - the "team/frontend-platform" label was applied to this issue.