Replace BTC by BCH in util.py error messages and docstrings

Summary: As per title

Test Plan:
Tweak wallet_basic.py line 31. Replace:

assert_fee_amount(fee, tx_size, fee_per_byte * 1000)

By:

assert_fee_amount(fee, tx_size, fee_per_byte * 10000)

Run:

./test/functional/test_runner.py wallet_basic

Check that there is an assertion (fee too low).
The assertion message should now contains a BCH amount instead of BTC.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2578