Configs: use a uniform syntax without Match exceptions (#507)

* Configs: use a uniform syntax without Match exceptions

The old style of specifying Configs used total functions.  The only way to
indicate that a key was not matched was to throw an exception.  Not only was
this a performance concern, but it also caused confusing error messages
whenever you had a match failure from a lookup within a lookup.  The
exception could get handled by an outer-lookup that then reported the wrong
key as missing.