gh-159786: Convert HACL* update() errors in `_hmac` after releasing the lock (GH-159805)

* gh-159786: Convert HACL* update() errors in `_hmac` after releasing the lock

`_hacl_hmac_state_update_once()` converted the HACL* exit code into an
exception while the thread state was detached and, for HMAC.update(),
while the object's mutex was held. On debug builds, this re-attached the
thread state under the mutex and then called PyErr_Occurred() with the
thread state detached in the chunking loop of `_hacl_hmac_state_update()`.

Return the raw HACL* exit code instead and convert it in `_hmac.new()` and
`_hmac.HMAC.update()` once the thread state is re-attached and the mutex
is released, as `hmac_digest_compute_locked()` already does. Release
builds keep ignoring the update() exit code.

Since `_hacl_convert_errno()` is now always called with an attached
thread state, remove the `PyGILState_Ensure()` added by gh-135561 and
assert that a thread state is attached instead.

* simplify ifdefs: always check for the hacl error

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Co-authored-by: Gregory P. Smith <greg@krypto.org>