[backport] gui: Show messages as text not html

Summary:
6fbc0986f gui: Show messages as text not html (Wladimir J. van der Laan)

Pull request description:

Currently, error messages (such as InitError) are displayed as-is, which means Qt does auto detection on the format.

This means that it's possible to inject HTML from the command line though e.g. specifying a wallet name with HTML in it. This isn't a direct security risk because fetching content from internet is disabled (and as far as I know we never report strings received from the network this way). However, it can be confusing.

So explicitly force the format as text.

This is a backport from Core PR12617 (https://github.com/bitcoin/bitcoin/pull/12617)

Test Plan:

ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4761