Fix config/flag priority and error handling - 1.1.9

Bug fixes

• Config values are no longer ignored. CLI flags with non-empty defaults
  (--smtp-port/-p, --tls-mode/-l) silently overrode config-file values
  even when not passed. They now default to unset, so config is respected;
  the built-in defaults (port 587, TLS mandatory) apply only when nothing is
  specified anywhere.
• Fixed a panic when the SMTP client failed to initialize — the error is
  now returned instead of dereferencing a nil client.
• Send failures now exit non-zero. sendEmail's error was previously
  discarded, so failures still reported success.
• Invalid config files and --body-file read errors now stop execution
  instead of silently continuing with empty values.
• Invalid --tls-mode values are rejected instead of silently degrading
  to the default TLS policy.

Note

Any script relying on a typo'd --tls-mode that previously "worked" via
opportunistic STARTTLS will now fail loudly — set a valid mode
(none, tls-skip, or tls).

New Contributors

• @ansiwen made their first contribution in #10

Full Changelog: 1.1.8...1.1.9