[Cashtab] Better validation for multisend txs

Summary:
This validation is currently handled in the Send.js component without CI unit testing. Improve test to return error line number. Add unit tests.

Remove tech debt + need this function for improved tx construction later.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14948