ptx: handle invalid regex arguments gracefully instead of panicking

[CrazyRoka]

Description

This PR fixes a panic in ptx when an invalid regular expression is passed to the -W (--word-regexp) argument.

Previously, running:
ptx -W 'bar\\\'
would result in a Rust panic (called Result::unwrap() on an Err value).

This change catches the regex::Error, wraps it in a USimpleError, and ensures the program exits with code 1 and prints a user-friendly message to stderr.

Testing

• Added unit tests in tests/by-util/test_ptx.rs to verify:
  • Trailing backslashes (bar\) return exit code 1.
  • Unclosed groups ((wrong) return exit code 1.
  • The stderr contains "invalid regular expression".

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/sort/sort-stale-thread-mem. tests/sort/sort-stale-thread-mem is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/sort/sort-debug-keys is no longer failing!

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/sort/sort-debug-keys is no longer failing!