[Cashtab] Validate multisend amounts for decimal places

Summary:
Cashtab does not currently validate send amounts for 2 decimal places in the multisend screen. Add this validation (will need it to ensure correct outputs are calculated with ecash-coinselect)

Update validation error msg to remove hardcoded value. Update unit tests to reflect function expectation of string.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack, PiRK

Reviewed By: #bitcoin_abc, emack, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14917