Remove impossible case

[aaronjensen]

According to dialyzer, this case is impossible. I would guess it has to do with what lookup/1 is capable of returning. It's not capable of returning anything that would trigger the case of extract_error_metadata/3 that would return nil.

endpoint.ex:53: The pattern {_, 'nil'} can never match the type {_,{atom(),binary(),_,_}}

It could be a dialyzer bug, there are a few nasty ones in 19.

[arjan]

By just looking at the code, I see a case where nil is returned from extract_error_metadata/3, namely when the plug_status < 500. In that case we don't report an error because non-500 HTTP errors are not logged to AppSignal.

I'll see if I can reproduce this with dialyzer.

[aaronjensen]

Fixed properly in #8 I believe.