Process recovery suggestion from Sparkle update error messages in UI alert

[zorgiepoo]

Brought up as one issue in discussion #2688

If there is a recovery suggestion in the update error, we will use that as the informative text, and the error description as the message text (instead of generic Update Error!)

Also remove an extraneous comma where it doesn't belong in the disk image error message.

Misc Checklist

• My change requires a documentation update on Sparkle's website repository
• My change requires changes to generate_appcast, generate_keys, or sign_update

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

Tested running app from disk image and checking for updates
Tested running app in translocated state and checking for updates
Tested running app in another language (Hebrew) and checking for updates from translocated state.
Tested running app in another language (Hebrew) and checking for updates from disk image.

macOS version tested: 15.2 (24C101)

[zorgiepoo]

Likely a side effect / fallout from #1516

[zorgiepoo]