There's a lot of demand for the Gitlab badges (#541) and the PR has been lingering, so I thought I'd start off one of the simple ones as a new-style service. This one is SVG-based, so it shouldn't require the API-token logic which could use some more testing and will require us to create an app and configure it on our server.
We don't have any validation in place for `queryParams`. Probably this should be added to BaseService, though for the time being I extracted a helper function.
Thanks to @LVMBDV for getting this work started in #1838!
* InvalidParameter: New error type
* Return inaccessible for 5xx errors from services
* Add test for Inaccessible on 5xx
* Add tests for named error types
Make a clear distinction between programmer errors ("internal errors") and runtime errors, and allow configuring the server to let the programmer errors bubble up in development and unit testing. This saves a huge amount of time because it generates ordinary stack traces when things go wrong. And, if these errors occur in production, we'll catch them, and display **shields | internal error** which is the equivalent of a 500 error.
