Ensure measurement not found returns status code 200

[corylanou]

Follow up to #1999

[otoolep]

All this string checking of errors feel hokey, but it's seems it's what we have right now. If it's works, +1.

@corylanou -- perhaps we need to revisit the errors at some point, to make better use of error types?

[corylanou]

@otoolep yes, I'm totally unhappy with this. It's about the most brittle way you can test this, and is likely to bite us again in the future. We probably need a better way of handling errors in the future. I would be a big fan of error types instead of vars as it gives us some more flexibility. However, if you are bubbling up errors, and format them to combine with another error, you will lose the type info anyway. I think more generically we need an error type, that has a message (string), and then a category, and if we want to bubble that up, we only change the message, not the category, or something like that.