Improve error output from Validate(), Translate(), and vmuxers

[mildwonkey]

Translate() errors which report an expected vs. actual values are sometimes, but not always, reversed:

validation failed, data is not an instance:
	schema expected `null`
		dashboard.json:18:13
		/scthema/dashboard_kind.cue:571:13
	but data contained `string`

I attempted to address this in #140 , which is when I learned that it's not always reversed.

[sdboyer]

updated title just to reflect that these particular errors are coming from the Validate() call that is made internally by vmuxers before they call Translate()

(Which also points to another issue, that the error model for thema - epic is #44 - needs to include a clear way of differentiating, presumably with errors.Is(), between different classes of errors that may occur during processing of input bytes to vmuxers)