Improve error message for API response when adding inactive user as a PR reviewer

Issue Summary

Using pullrequest update API endpoint, to add a reviewers to the existing Pull request, if you include UUID of a user that is set to inactive, our API responds with 

"message": "reviewers: Malformed reviewers list"

Steps to Reproduce

1. Create a Pull request on Bitbucket
2. Make sure the user who you would like to add has access to this repository
3. Deactivate user's account via Active Directory (for managed account)
4. Use the inactive user UUID to update the PR and try to add him as a reviewer

https://developer.atlassian.com/bitbucket/api/2/reference/resource/repositories/%7Bworkspace%7D/%7Brepo_slug%7D/pullrequests/%7Bpull_request_id%7D#put

   5. Bitbucket will respond with not a very user friendly message that might lead to thinking that the syntax for JSON was wrong

Expected Results

To respond to the user indicating: "User you are trying to add as a reviewer is inactive, thus this action cannot be completed"

Actual Results

{"type": "error","error": {"fields": {"reviewers": [  "Malformed reviewers list"],},"message": "reviewers: Malformed reviewers list"}

Workaround

The workaround for the user is to go Bitbucket UI and check if the user he is trying to add as a reviewer is Active or not. This can also be checking by making a call to the user UUID endpoint and see if the user status is Active or Not.

Then remove that user's UUID from the PUT request body to complete the PR update successfully.