Change errors export from Descriptors to ErrorDescriptors
Merge errors and urls package into unified v2 package

To provide a single package with v2 API definitions, the locked down portions
of the API have been merged into a single package. References to exported items
will appear as v2.XXX, indicating their origin. The definitions in this package
will soon be locked down for change, acceppting only additions that will not
change protocol behavior.