assign error, don't return it from catch block

Always format the result!
If we call the 'rest' endpoint without json param
we are getting the error in json format
instead of the expected xml format.

Signed-off-by: Kevin Cristiano <kcristiano@kcristiano.com>

wp-rest/Controller/Rest.php

@@ -78,7 +78,7 @@ class Rest extends Base {
 
 		} catch ( \CiviCRM_API3_Exception $e ) {
 
-			return $this->civi_rest_error( $e->getMessage() );
+			$items = $this->civi_rest_error( $e );
 
 		}