Handle missing Flatpaks gracefully

[wjt]

Previously, if a listed Flatpak app or runtime is not present in that
remote, remote.match() would return None, but the calling code would
assume it is not None. This leads to a failure like this:

+ 20:16:29 INFO eibflatpak: Looking for app ref "com.orama_interactive.Pixelorama" match in flathub
+ 20:16:29 INFO eibflatpak: Removing repo option core.xa.languages
+ 20:16:29 INFO eibflatpak: Removing repo option core.xa.extra-languages
+ 20:16:29 INFO eibflatpak: Removing repo option core.xa.masked
Traceback (most recent call last):
  File "/tmp/image-build-IZAJlTeyep/eos-image-builder/hooks/content/50-flatpak", line 61, in <module>
    manager.pull()
  File "/tmp/image-build-IZAJlTeyep/eos-image-builder/lib/eibflatpak.py", line 837, in pull
    txn = self._new_transaction()
  File "/tmp/image-build-IZAJlTeyep/eos-image-builder/lib/eibflatpak.py", line 808, in _new_transaction
    self._add_installs(txn)
  File "/tmp/image-build-IZAJlTeyep/eos-image-builder/lib/eibflatpak.py", line 799, in _add_installs
    logger.info('Adding app %s from %s', ref.ref, remote.name)
AttributeError: 'NoneType' object has no attribute 'ref'

Handle this gracefully, collecting all errors in case more than one
Flatpak is missing, then fail at the end.

https://phabricator.endlessm.com/T34603

[wjt]

Draft because totally untested.

[dbnicholson]

Looks good to me! I'd be comfortable merging this as is if you don't have time to test.

[wjt]

Tested this with 1 found and 2 not-found refs, it looks good:

+ 21:40:44 INFO eibflatpak: Looking for app ref "org.freedesktop.Bustle" match in flathub
+ 21:40:44 INFO eibflatpak: Adding app app/org.freedesktop.Bustle/x86_64/stable from flathub
(process:49709): flatpak-DEBUG: 21:40:44.363: Fetching summary index file for remote ‘flathub’
(process:49709): flatpak-DEBUG: 21:40:44.363: Loading https://dl.flathub.org/repo/summary.idx using curl
(process:49709): flatpak-DEBUG: 21:40:44.428: Received 10001 bytes
(process:49709): flatpak-DEBUG: 21:40:44.452: Loaded indexed summary file 0085996624446d6a2648cfe204cdca177355784c27bf916681c001b4f3d33e2d from cache for remote ‘flathub’
(process:49709): flatpak-DEBUG: 21:40:44.452: Transaction: install flathub:app/org.freedesktop.Bustle/x86_64/stable[*]
+ 21:40:44 INFO eibflatpak: Looking for app ref "org.freedesktop.Hustle" match in flathub
+ 21:40:44 ERROR eibflatpak: app org.freedesktop.Hustle not found from flathub
+ 21:40:44 INFO eibflatpak: Looking for app ref "org.freedesktop.Rustle" match in flathub
+ 21:40:44 ERROR eibflatpak: app org.freedesktop.Rustle not found from flathub
+ 21:40:44 INFO eibflatpak: Removing repo option core.xa.languages
+ 21:40:44 INFO eibflatpak: Removing repo option core.xa.extra-languages
+ 21:40:44 INFO eibflatpak: Removing repo option core.xa.masked
Traceback (most recent call last):
  File "/sysroot/home/wjt/src/endlessm/eos-image-builder/hooks/content/50-flatpak", line 61, in <module>
    manager.pull()
  File "/sysroot/home/wjt/src/endlessm/eos-image-builder/lib/eibflatpak.py", line 854, in pull
    txn = self._new_transaction()
          ^^^^^^^^^^^^^^^^^^^^^^^
  File "/sysroot/home/wjt/src/endlessm/eos-image-builder/lib/eibflatpak.py", line 825, in _new_transaction
    self._add_installs(txn)
  File "/sysroot/home/wjt/src/endlessm/eos-image-builder/lib/eibflatpak.py", line 818, in _add_installs
    raise FlatpakError(
eibflatpak.FlatpakError: Could not find listed flatpaks: org.freedesktop.Hustle, org.freedesktop.Rustle