[Cashtab] Prevent content jumping on receive screen on input switch with tighter validation msgs

Summary:
If the user switches from XEC to FIRMA, they see a validation error bc 0 is not allowed. This is annoying but maybe necessary with current workflow, as bip21 requires SOME value there. So we would need to pick a default to avoid the error.

The real annoying thing though is the content jumping.

Trying to solve this, saw a lot of these validation errors are overly long and complex. Not really useful for the user so much as dev notes.

So, tighten up the validation msgs to fix the content jumping and improve concise UX. And add some space for the validation error to appear without causing content jumping.

Test Plan: npm test, npm start and try it out

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18812