do-release-tags fails if reference doesn't exist

[ajodock]

In do-release-tags if the new reference doesn't exist the user gets a cryptic error.

Ref centos-atomic-host/7/x86_64/devel/alpha does not currently exist
Traceback (most recent call last):
  File "./do-release-tags.orig", line 82, in <module>
    _,promotedcommit = r.write_commit(newparent, '', body, promotedmetadict.end(), newcommitroot, None)
NameError: name 'newparent' is not defined

The first line tells the user what they need to know but the exception isn't very helpful. It would be nice if the script would exit before the exception, and if the user had the option to automatically create the reference.

Automatically creating the reference could be useful for build systems that would auto generate the YAML file which may dynamically create new references.