LSIF: Remove noisy NoLSIFDumpError

Created by: efritz

Throwing NoLSIFDumpError causes instrumented blocks to emit the exception. This makes it very noisy (and alarming) in prod where there are no actual errors, but the /exists endpoint working normally.

This returns undefined database/dump in place of a thrown exception. This has the (intended) consequence of emitting a warning only in places that don't expect such a condition (the endpoints that are meant to be called after checking if code intelligence data exists).

A secondary consequence changes the behavior of /request endpoints by returning a 200 when no intelligence data is found where it used to return a 500. I think this is an improvement, but we could do even better with a specialized error message or status code to indicate that the request was not called with the correct ordering (or was called in the presence of a race condition such as a dump being deleted).