app: Print a warning if xa.metadata doesn't exist

All flatpaks built using version 0.9.4 or newer should have the
xa.metadata field in the commit metadata, so warn if it doesn't exist.

This commit changes the info command to print a warning rather than
nothing and changes the info-remote command to print a warning rather
than error out.

Closes: #1351
Approved by: alexlarsson
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f