BUG-1027: emit more diagnostics when augmentation is not found

This adds more stern warnings when we fail to find augmentation targets.

Change-Id: Iec338917e7f175c4e977529761f1e8fa272d0bb1
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 2952d48e853a70d20a59fe427a37280f9131d452)
Merge "BUG-1027: emit more diagnostics when augmentation is not found" into stable/hydrogen