Show clear signup error for usernames with underscores

[guaka]

Summary

• reject underscores earlier during signup
• return a clear username-specific error message for underscore usernames
• update signup validation coverage for underscore usernames

Validation

• Server test task was attempted, but the local run did not reach tests because the native mmmagic module failed to load under the current Node runtime (ERR_DLOPEN_FAILED).

[guaka]

relates to #2723

[guaka]

let's try this simpler way first: d8fc934

[chmac]

Definitely seems like an easier option!