When switching on an error, using the captured value instead of the original
one is always preferable since its error set has been narrowed to only
contain errors which haven't already been handled by other switch prongs.
The subsequent commits will disallow this form as an unreachable `else` prong.
This "get" is useless noise and was copied from FixedBufferWriter.
Since this API has not yet landed in a release, now is a good time
to make the breaking change to fix this.
Justus Klausecker