Give useful information to the KPIs when reporting a custom error.

* If reporting a custom error type, use the action.title field as the reported error.

fixes #3584
Merge pull request #3585 from mozilla/issue_3584_kpi_error_title

Give useful information to the KPIs when reporting a custom error.

@kparlante gave me an r+, merging. Thanks @kparlante and @seanmonstar!