[GENERIC viewer] Display a warning message, using the errorWrapper, for documents with signatures (PR 13214 follow-up, issue 13215)

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.

---
[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.

web/l10n_utils.js

@@ -81,6 +81,8 @@ const DEFAULT_L10N_STRINGS = {
   printing_not_ready: "Warning: The PDF is not fully loaded for printing.",
   web_fonts_disabled:
     "Web fonts are disabled: unable to use embedded PDF fonts.",
+  unsupported_feature_signatures:
+    "This PDF document contains digital signatures. Validation of signatures is not supported.",
 };
 
 function getL10nFallback(key, args) {