Record skipped MR import and make available via API

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When investigating #463562 (closed), we found that merge requests failed to import when the following conditions were met:

• source_project is nil on the importing MR.
• The importing MR has an invalid diff.

At the moment this results in an unhandled exception fetching the MR ref:

With no source project, and no diff, the MR can show only discussion, and no change. With no way to recover from this, or other types of mangled import data, it makes sense to treat this as a "skipped" import, rather than a "failed" import.¹ Instead, we should:

• Catch errors raised in this scenario
• Skip importing this MR.
• Record an ImportFailure on the importing project
• Make this data available to the front-end
• Update the excluded items documentation

This will facilitate a front-end change where we include skipped MRs (as opposed to failed MRs) in the post-import statistics.

1. #463562 (comment 2211112935) ↩