Updating PR descriptions gets 400 Bad Request if the reviewer has left / become inactive

Issue Summary

This is reproducible on Data Center: Unsure.

Steps to Reproduce

1. Create a PR
2. Add a reviewer
3. Make that reviewer user inactive / deleted
4. Try to update the PR description

Expected Results

Updating the PR description succeeds. 

Also, provide meaningful error message in the UI to self-serve the users.

Actual Results

The update fails every time, with an HTTP 400 "Bad request" error. Error message: 
"Malformed reviewers list".
Example HTTP request that fails:

PUT https://bitbucket.org/!api/2.0/repositories/atlassian/barrel/pullrequests/11727 

Workaround

Remove the inactive reviewer, and then updates will succeed.