[backport#15894 2/3] Add MSG_NOPREFIX flag for user messages

Summary:
It forces do not prepend error/warning prefix.

https://github.com/bitcoin/bitcoin/pull/15894/commits/96fd4ee02f6c3be21cade729b95a85c60634b0f8

Depends on D7124

Partial backport of Core PR15894

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7125