add better message when patch fails to apply, currently 'abort: uncommited changes'

[Archaeopteryx]

Code review bot failed to run for https://phabricator.services.mozilla.com/D285267 which had been developed standalone but afterwards had been configured to have a new parent revision: https://phabricator.services.mozilla.com/D285157
Between the initial base revisions of those two changes, https://phabricator.services.mozilla.com/D284354 had landed and modified code adjacent to a change in D285267.

moz-phab patch D285267 applied D28517 first and used it as base for D285267 which effectively rebased it. Because of the changes from D284354 missing, the patch failed to apply.

Code review bot posted to Phabricator's D285267 two comments:

WARNING: The code review bot failed to apply your patch.

abort: uncommitted changes

Changes:

Warn directly the change will likely fail to land

Suggest a rebase in case the developer has manually edited the parent revision.