Add error handling for \Drupal\package_manager\Stage::getIgnoredPaths()

Testing Drupal\Tests\package_manager\Kernel\StageTest
.......................F.                                         25 / 25 (100%)

Time: 00:39.430, Memory: 6.00 MB

There was 1 failure:

1) Drupal\Tests\package_manager\Kernel\StageTest::testFailureCollectIgnoredPaths
Failed asserting that exception message 'No composer.json file can be found at vfs://root/active
' contains 'Composer could not find the config file: vfs://root/active/composer.json
'.

See #3315834-46: GitExcluder should not ignore .git directories that belong to packages installed by Composer why this change does not make sense to land separately: some excluder is needed to trigger that case, so we might as well land this as part of #3315834.

This needs polish. 3 remarks on the MR. I like how the scope of this issue is far more manageable than the issue where this was discovered though 😄

Added to #3319030: Drupal Core Roadmap for Package Manager and Update Manager.

A few remarks to ensure the MR is clear — but we're definitely almost done!

The hardening was moved to #3335766: Harden LockFileValidator, add stopPropagation() at failures, excellent! Added it to the roadmap: #3319030-49: Drupal Core Roadmap for Package Manager and Update Manager.

Thanks!