Add debug statements to PushImage build api endpoint

BUG=chromium:1122854
TEST=unit tests

Change-Id: Ia73bd02610151fdd6afeadd203eb5c42fbdc50ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2659576
Commit-Queue: Jack Neus <jackneus@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Jack Neus <jackneus@google.com>
Auto-Submit: Jack Neus <jackneus@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/api/controller/image.py b/api/controller/image.py
index 0a75bad..7c3f8b3 100644
--- a/api/controller/image.py
+++ b/api/controller/image.py
@@ -317,4 +317,5 @@
         **kwargs)
     return controller.RETURN_CODE_SUCCESS
   except Exception:
+    logging.error('PushImage failed: ', exc_info=True)
     return controller.RETURN_CODE_COMPLETED_UNSUCCESSFULLY