Fix AttributeError being raised in buckets controller

This PS fixes an AttributeError raised in buckets controller
when Deckhand tries to parse a malformed YAML which in turn
causes Deckhand to attempt to raise a 400, but accesses an
invalid attribute in the exception object, causing the
AttributeError.

Example error:

    AttributeError: 'ScannerError' object has no attribute 'format_message'

Change-Id: I54865e4830d3d34e813b1ecfa3105cf6243f2ca0

releasenotes/notes/fix-attribute-error-in-bucket-controller-b5c9e410823abd19.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Deckhand will no longer throw an ``AttributeError`` after a
+    ``yaml.scanner.ScannerError`` is raised when attempting to parse a
+    malformed YAML document. Deckhand should now correctly raise a
+    "400 Bad Request" instead.