Match CBOR errors via shape name instead of shapeId

[Madrigal]

Issue #, if available:

Description of changes:
There's an ongoing discussion about the right way to match errors on CBOR. This would be one way of checking only against shape name instead of against shapeId. Don't merge this until the discussion is finalized

Diff would look like (typ is the full shape id)

-     switch string(typ) {
-     case "com.amazonaws.cloudwatch#ResourceNotFound": 
+     // namespace can be mangled by service, so matching by error shape name
+     errorParts := strings.Split(typ, "#")
+     errorName := errorParts[len(errorParts)-1]
+     switch string(errorName) {
+     case "ResourceNotFound": 

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[wty-Bryant]

nit: is this new function used anywhere?