Exit with non-zero code when database is not accessible

[OdenTakashi]

Problem

AnnotationDecider#annotate? has a catch-all rescue => e that returns false for any StandardError, including database connection errors. While error messages are printed to stderr, the command still exits with code 0 because the errors are treated as "skip this file" rather than a fatal condition. This is problematic for CI workflows using --frozen, where a non-zero exit code is expected on failure.

Solution

Add a specific rescue for ActiveRecord::ConnectionNotEstablished and ActiveRecord::NoDatabaseError before the catch-all handler. These errors now abort with a clear message and exit code 1, instead of being swallowed by the catch-all and treated as a skippable file-level issue. Other errors (e.g. model syntax errors, LoadError) continue to be rescued and skipped as before.

Follow-up

exe/annotaterb already has a # TODO: Return exit status comment, and Runner#run does not currently return a meaningful exit status.

As a follow-up, it may make sense to centralize error handling at the Runner or exe/annotaterb level. For example, the entire execution could be wrapped in a single top-level rescue that catches errors, prints a user-friendly message, and exits with a non-zero status. This would allow error handling to be managed in one place, rather than requiring specific exceptions to be handled in lower-level code such as AnnotationDecider.

ridgepole takes this approach.

However, that would move this change closer to the broader error-handling design, so it has been kept out of scope for this PR.

refs: #240