cue/errors: convert more errors to cue/errors.Error

Change-Id: Iecd21974252dfac0ff1dd72e1ce40ddf107ee444
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2368
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

cue/errors/errors.go

@@ -198,7 +198,10 @@ func (e *posError) Cause() error                { return e.err }
 
 // Error implements the error interface.
 func (e *posError) Error() string {
-	return e.Message.Error()
+	if e.err == nil {
+		return e.Message.Error()
+	}
+	return fmt.Sprintf("%s: %s", e.Message.Error(), e.err)
 }
 
 // List is a list of Errors.

cue/types.go

@@ -182,7 +182,7 @@ func unwrapJSONError(err error) errors.Error {
 	case errors.Error:
 		return &marshalError{x}
 	default:
-		return &marshalError{errors.Wrapf(err, token.NoPos, err.Error())}
+		return &marshalError{errors.Wrapf(err, token.NoPos, "json error")}
 	}
 }