Charm now validates YAML-loaded Actions schema

At the time the Charm's Actions schema is loaded from YAML, gojsonschema
now uses its Validate method to ensure that the loaded schema conforms to
JSON-Schema Draft 4.  The schema for JSON-Schema Draft 4 has been added
as a .json file to the charm package in order to avoid remote queries to the spec.
This looks like it could end up highly verbose.
How about producing an error that looks like this?

    cannot validate JSON schema: error1; error2; error3

alternatively (this is an approach I've seen taken in other places, such as the go parser),
just include the first error and an indication that there are more.

   cannot validate JSON schema: error1 (and 99 more errors)