[Cashtab] Catch any errors that come up in creating the agora partial acceptTx

Summary:
I have managed to hit this in dev / testing for getting the right fee calc (see D18380)

Add a try catch and a toast in case we have any errors trying to accept tx. We aim to have validation prevent this, but there could always be edge cases, esp as we look into getting right on the line with max buys.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D18381