restapi: No NPE when no linked exception

Some exceptions generated when posting an incorrect XML document don't
contain a linked exception. In those cases we try to reference them and
thus a NPE is produced, for example, if we post a valid but incorrect
XML document to a collection:

  # curl \
  --insecure \
  --request POST \
  --header "Accept: application/xml" \
  --header "Content-Type: application/xml" \
  --user "${user}:${password}" \
  --data "<garbage/>" \
  ${url}/tags

To avoid this issue this patch adds a check to the XML message body
reader, so that when these type of exceptions occur it will generate the
expected usage message instead of the NPE.

Change-Id: I7158c3d5c9d3c928209444d27c369e8753b2c840
Bug-Url: https://bugzilla.redhat.com/1091981
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>