The mc_project_get_issues() returns "project doesn't exist" when it is passed 0 (ALL_PROJECTS) as the project id. This is because it internally uses project_exists() without first checking that the project is not ALL_PROJECTS.

We should search for all usage of project_exists() and make sure that the same issue doesn't apply to other APIs. For example, given that we now have global categories, we may want to add support for ALL_PROJECTS for such APIs.