To simplify the overall implementation, given that it only applies to the GENERIC-viewer, this patch purposely re-uses the existing `errorWrapper`-functionality to display the message.
While that one is mostly intended for actual *errors*, by re-using it here we considerably reduce the amount of code/complexity necessary for supporting this new warning. It's obviously possible to re-factor/improve this later on, but the patch should do just fine here since it'll indeed inform users (of the GENERIC-viewer) about unverified signatures.
Finally this patch also tweaks the background-color of the `errorWrapper`, making it 20 percent lighter respectively darker (depending on the theme) to make it "stand out" a little bit *less*.[1] While it may perhaps be useful to re-style/re-factor the `errorWrapper`, this patch probably isn't the right place for doing that.
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[1] Note how in the MOZCENTRAL-viewer, which instead uses the browser notification-bar, we're purposely using a neutral colour to not draw too much attention to the notification-bar.
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