gui: Show messages as text not html
gui: Show messages as text not html

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.

Github-Pull: #12617
Rebased-From: 6fbc0986fa2d49a1cb65b60eca71c25c84842a54

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