[backport#15894 1/3] Prepend the error/warning prefix for GUI messages

Summary:
https://github.com/bitcoin/bitcoin/pull/15894/commits/f0641f274ffe94fbe7cae43c07a9373013739df2

Partial backport of Core PR15894

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7124