Add details to MigrateSkipRowException messages caused by stub creation

@dsdeiz, Sorry, I do not recall what I did to force the error.

I took another look here and then went ahead and added the change I proposed in #7. I also updated \Drupal\Tests\migrate\Unit\process\MigrationLookupTest::testTransformWithStubbing to use a data provider with two cases for this change.

This came up as a random #bugsmash triage target, which is where the new life is coming from. Mostly looks fine. Added a few MR suggestions for some nits.

Tanuj. → made their first commit to this issue’s fork.

Had to run locally as I couldn't run the test-only feature.

Appears this has already gone through a few rounds of reviews and appears the feedback has been addressed.

Appears to be one out of scope change in the MR.

Applied my own suggestion to save time here.

Committed and pushed to 11.x and 10.3.x. Did not backport cleanly to 10.2.x, this is only a minor improvement so not sure it is worth backporting there.

longwave → closed merge request !6764

Automatically closed - issue fixed for 2 weeks with no activity.