Update invokeProcessAction.md

Clarifies https://github.com/MicrosoftDocs/powerapps-docs/issues/601 a bit. It's a little vague because the object structure can change depending on the particular process action used.

Author: stjanovi

powerapps-docs/developer/model-driven-apps/clientapi/reference/Xrm-Utility/invokeProcessAction.md

@@ -37,7 +37,8 @@ For more information about actions, see [Use actions](/powerapps/maker/common-da
 
 ## Returns
 
-On success, returns Web API result along with any action output.
+On success, returns an object with the Web API result along with any action output.
+On failure, returns an object with error details.
 
 ### Related topics
 [Use actions](/powerapps/maker/common-data-service/actions)