Improve --config error when file not found

[rdmarsh]

Problem

When --config is passed a file that does not exist, elm falls through to a
generic credentials error:

Error: access_id, access_key or account_name not set via cli or config file
Default config file: /home/user/.config/logicmonitor/credentials/config.ini

This is misleading — the user passed an explicit path and the real problem is
that the file was not found, not that credentials are missing.

Expected behaviour

A clear error pointing at the missing file, similar to what --profile does:

Error: config file not found: /home/user/.config/logicmonitor/credentials/bob.ini

Notes

• This is a click_config_file behaviour — it silently ignores a missing config
  file rather than erroring.
• Fix likely belongs in _capture_config_path callback in _jnja/elm.py.j2:
  check os.path.exists(value) and exit with a clear message if not.
• --profile already does this correctly for its resolved path.

🤖 Generated with Claude Code