pw_software_update: Replace macro with function

Reduce code size by moving some parts of the SET_ERROR() macro into a
function. This saves 2 * 26 PW_CHECK() invocations by pushing those
checks into shared code.

More savings are possible by re-using the computed log slug, but that
requires adding functionality to pw_log and will come later.

Change-Id: I191077b3ce79b5578501222250f8620ec8b38b9d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/70120
Commit-Queue: Yecheng Zhao <zyecheng@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>