Debug impl on Error hides the source error codes #69
Description
The Debug impl on the multer::Error type can hide the underlying error code, and generally follows Display formatting instead of the usual Debug formatting, which is very annoying when trying to debug issues. In my case, I'm having some problems while reading from the stream, and would like to read the debug impl of the underlying error in StreamReadFailed, but multer only prints the display impl, which just unhelpfully prints "failed to read stream".
Why not just slap #[derive(Debug)] on the multer::Error type? It looks like it'd just work.