compress/flate: make huffmanBitWriter errors persistent
For persistent error handling, the methods of huffmanBitWriter have to be
consistent about how they check errors. It must either consistently
check error *before* every operation OR immediately *after* every
operation. Since most of the current logic uses the previous approach,
we apply the same style of error checking to writeBits and all calls
to Write such that they only operate if w.err is already nil going
into them.
The error handling approach is brittle and easily broken by future commits to
the code. In the near future, we should switch the logic to use panic at the
lowest levels and a recover at the edge of the public API to ensure
that errors are always persistent.