Report invalid --format-options as a CLI error, not a traceback

[VXNCXNX]

An invalid --format-options value exits with an unhandled argparse.ArgumentTypeError traceback instead of the normal HTTPie error:

$ http --offline --format-options=json.indent:x pie.dev/get
  File "httpie/cli/argtypes.py", line 244, in parse_format_options
argparse.ArgumentTypeError: invalid value 'x' in 'json.indent:x' (expected int got str)

Same for an unknown section (--format-options=json) or key (--format-options=json.nope:1). _process_format_options calls parse_format_options outside argparse's own type machinery, so the error escapes to the top level. It is now caught and routed through self.error, which is what _guess_method and _parse_items already do a few lines above; --traceback still re-raises, matching those two.

After the fix the same command prints error: --format-options: invalid value 'x' ... and exits 1. Three parametrized cases added in tests/test_output.py. Suite is green apart from two pre-existing big5 charset failures in test_encoding.py, identical on an unmodified checkout: 1020 passed, 2 failed.

AI disclosure: written with Claude Code. I ran the CLI before and after and checked the mutation myself.