[Cashtab] [final countdown bch param p3] Change name of send tx error code object

Summary:
T2730

Depends on D12807

Change name of global error code object SEND_BCH_ERRORS to SEND_XEC_ERRORS

Test Plan:
Confirm this is only a name change
grep -r SEND_BCH_ERRORS src/ has no result

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12808