[Enhancement]: Improve communication of what the snap can do and cannot do.

[brlin-tw]

What changes would you like?

We should communicate better the fact that the wormhole snap cannot access the system's /tmp directory (for good reasons).

The proper way to do so might be:

• Specify the limitation on the snap listing.
• Implement launcher/patch to:
  • check and print warning whenever the user tries to access files outside of snap's confinement:
    • /home/$USER/*
    • /media/*
    • /mnt/*
  • print a single-time notice message/dialog regarding the limitation.

Any extra information?

Refer to https://github.com/snapcrafters/magic-wormhole/pull/16#issuecomment-2399698607 for the context.

[abitrolly]

Error messages should be actionable. For example, propose user to choose dir where to save incoming file if current location is not writable.