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.
In general when we include arbitrary text in error messages, we'll quote it.

e.g.
return nil, fmt.Errorf("bad param name %q", paramName)

That particularly applies when something has failed validation.