Fix error location

[bovee]

With the big v0.3 rewrite I changed how the parser advances so the EtErrorContext::byte in most error messages is no longer pointing to the correct position in the record, but rather to the first byte.

I think the best fix for this is to manually set byte (maybe in EtError::new?) when the error is created and to make sure that the calling next function doesn't overwrite this when it's handling the error, but this involves updating every EtError constructor.