Capture Rails runner's exceptions before exiting

[st0012]

Rails currently doesn't provide any callback for capturing Rails runner's exceptions, so we need to use at_exit for it. But it should be supported in 7.1 (PR: rails/rails#44999) via the enhanced error reporter integration.

And because it's very complicated, if not impossible, to monitor if a Rails runner process reports to Sentry, so I didn't add any test for it. I used this for testing instead:

bundle exec rails runner "1/0" # should be reported
bundle exec rails runner "1/1" # doesn't do anything

Implements and closes #1789

[codecov-commenter]

Codecov Report

Merging #1820 (ab8561f) into master (4ba0ca2) will increase coverage by 1.88%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1820      +/-   ##
==========================================
+ Coverage   96.58%   98.46%   +1.88%     
==========================================
  Files         120      145      +25     
  Lines        7373     8661    +1288     
==========================================
+ Hits         7121     8528    +1407     
+ Misses        252      133     -119     

Impacted Files	Coverage Δ
sentry-rails/lib/sentry/rails/railtie.rb	97.33% <100.00%> (+71.30%)	⬆️
...rails/tracing/action_controller_subscriber_spec.rb	100.00% <0.00%> (ø)
...entry/rails/tracing/action_view_subscriber_spec.rb	100.00% <0.00%> (ø)
sentry-rails/spec/sentry/rails/event_spec.rb	100.00% <0.00%> (ø)
sentry-rails/spec/sentry/rails/client_spec.rb	100.00% <0.00%> (ø)
...s/lib/sentry/rails/overrides/streaming_reporter.rb	61.53% <0.00%> (ø)
...entry-rails/spec/sentry/rails/action_cable_spec.rb	100.00% <0.00%> (ø)
...ails/breadcrumb/monotonic_active_support_logger.rb	100.00% <0.00%> (ø)
...entry-rails/lib/sentry/rails/controller_methods.rb	100.00% <0.00%> (ø)
...y-rails/lib/sentry/rails/controller_transaction.rb	100.00% <0.00%> (ø)
... and 29 more

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