[Cashtab] Standardize mnemonic validation error msg

Summary:
Some mobile devices can capitalize the first word of a seed when it is typed in.

We prevented this with "type="email"" before but lost that when we upgraded the inputs.

Restore the prevention and improve the validation phrase.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17278