Add HeaderSizeError for downstream consumers to match on

[Daenyth]

When raising header size mismatches, use this specific error.

It will cause, for example, exception reporting software like Sentry to
correlate these exceptions separately from logical errors like a bad Decoder

This should be backwards compatible since it's a subclass

[satabin]

While we are at it, could it make sense to also have two extra fields actual and expected?

[Daenyth]

I thought about that, but it wasn't clear to me that it was useful beyond just knowing that it happened. But it's cheap to add, so sure.

[satabin]

LGTM