Stop sending client (4xx) and upstream (502, 504) errors to Sentry #21

We currently send information about every kind of error we currently encounter to Sentry, including errors that were not internal (i.e. bad requests by the client or upstream errors from the requested URLs to diff). Those non-internal errors are “expected” in the sense that they don’t represent a problem with the differ, and probably don’t make sense to send to Sentry. For EDGI’s case, they also represent the vast majority of what we send to Sentry, which is problematic because it can push us towards quota limits.

These kinds of errors are probably better handled with logs and with metrics than with Sentry, and so we probably shouldn’t send them to Sentry. If other deployments need them (e.g. Internet Archive), we could probably add an environment variable to control it. (Maybe SEND_EXTERNAL_ERRORS?)