Fix exit in generate_header.py and some formatting nits

Summary:
Possible exit solutions are sys.exit() or os._exit(), but
os.exit() doesn't exist. Using sys.exit() incidentally removes the
dependency to os.
Also fix some spaces in the generated code so it matches our
clang-format rules, as it is trivial and saves as much as 3 bytes.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3418