Fix cli.py --html traceback crash and add error-path test covera

[evanjain-dot]

html previously crashed with a raw traceback if its output
directory didn't exist; now catches OSError like every other
error path in this file (closes #268).

Added tests/test_cli.py coverage for every branch make coverage
flagged as untested: --map/--cross/--reference/--proxy-hints
error handling and both --html write branches (closes #269).

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[github-actions]

@yakew7 @ahmdkaml @Shreyash0712 - new PR to review.

[propcgamer20-png]

i approve

[Shreyash0712]

Looks good to me as well. @yakew7

Minimal changes and solves the bug. Though my agent did raise a flag -

Currently, to_html(result) is evaluated while the file is already open for writing. If to_html(result) were to ever throw an unexpected exception (e.g., a ValueError or TypeError during formatting), the script would crash, but it would leave behind an empty 0-byte file on the user's disk since open() already succeeded.

Generating the HTML string right before opening the file avoids this edge case entirely:

        if args.html:
            html_content = to_html(result)
            try:
                with open(args.html, "w", encoding="utf-8") as fh:
                    fh.write(html_content)
            except OSError as exc:
                print(f"error: could not write HTML report to {args.html}: {exc}",
                      file=sys.stderr)
                return 2
            print(f"HTML report written to {args.html}", file=sys.stderr)

(The exact same logic applies to the compare command section below it with compare_to_html(result)).

Again, this is a super minor edge case since the formatting functions shouldn't be crashing anyway, but it's a good pattern to keep I/O operations as isolated as possible.

[yakew7]

Good catch @Shreyash0712 - implemented in 449ab5a: to_html()/compare_to_html() now run before the file is opened, so a formatting exception can't leave a truncated 0-byte report behind.