Do not report exceptions rescued by ActionDispatch

[AlanGabbianelli]

After upgrading to rails 6, we started seeing this error:
Mime::Type::InvalidMimeType "charset=utf-8" is not a valid MIME type
Example here:
https://sentry.io/organizations/govuk/issues/1549200925/?end=2020-03-06T12%3A00%3A23&environment=staging&project=202225&query=is%3Aunresolved+url%3Ahttps%3A%2F%2Fwww-origin.staging.govuk.digital%2Freport-an-unregistered-trader-or-business&start=2020-03-05T16%3A07%3A23&utc=true

This is because, even though these errors are rescued by ActionDispatch,
they still reach Sentry. Here's the explanation from Sentry's docs:

Rails catches exceptions in the ActionDispatch::ShowExceptions or
ActionDispatch::DebugExceptions middlewares, depending on the
environment. When rails_report_rescued_exceptions is true (it is by
default), Raven will report exceptions even when they are rescued by
these middlewares.

So, in order to stop this behaviour, we can set this option to be false.

Related PRs:

• Stop reporting exceptions rescued by ActionDispatch email-alert-api#1138
• Exclude Mime::Type::InvalidMimeType exception govuk_app_config#137

Trello card: https://trello.com/c/Ik7ulDXQ/1768-3-upgrade-frontend-to-rails-6

[thomasleese]

Do we need both this change here and alphagov/govuk_app_config#137? It seems like we're likely to want to set rails_report_rescued_exceptions = false across all our applications, so should we set that in govuk_app_config instead. Presumably then we don't need to have so many Rails exceptions in the excluded_exceptions.

[kevindew]

Oh this config option looks neat!

Toms suggestion to set this in govuk_app_config sounds great.

If you need a list of the rails default rescue responses they’re in this doc: https://guides.rubyonrails.org/configuring.html#configuring-action-dispatch

[AlanGabbianelli]

@thomasalanlee both changes have the effect of Mime::Type::InvalidMimeType not showing up in Sentry but they are different:

• Do not report exceptions rescued by ActionDispatch #2264 will act specifically for frontend and will stop all exceptions rescued by rails (not only Mime::Type::InvalidMimeType) from appearing in Sentry (this app - all errors)
• Exclude Mime::Type::InvalidMimeType exception govuk_app_config#137 will act for all apps that use govuk_app_config and will stop only Mime::Type::InvalidMimeType from appearing in Sentry (all apps - this error)

I think your suggested approach is better. If we take the config that acts on all errors and apply it to the gem that acts on all apps, we can stop all exceptions rescued by rails from appearing in Sentry for all apps that depend on govuk_app_config.

I'll close #2264 and alphagov/govuk_app_config#137 and open a new PR for that.

[AlanGabbianelli]

Closing this in favour of alphagov/govuk_app_config#138