[ews] Add logging when FindModifiedLayoutTests fails to access the PR content.
Pull request: https://github.com/WebKit/WebKit/pull/12986
Committed 263193@main (d01d57322f19): <https://commits.webkit.org/263193@main> Reviewed commits have been landed. Closing PR #12986 and removing active labels.
<rdar://problem/108334928>