Follow-up from "Productivity Analytics: Add error handling for reporting on groups which have no plan"

The following discussion from !15291 (merged) should be addressed:

• @wortschi started a discussion: (+1 comment)

  Initially, we stored a Boolean on hasError (since we didn't care about the error code). Now, with this change we're setting the error code on the state. The state's property name hasError might be misleading since it could be set to a number. Might be better to change the property name to errorCode. This could be done in a follow-up issue since we would need to update a bunch of references. Thoughts?

Let's rename the state's hasError property to errorCode.