Make event.unhandled overridable for JVM errors

[fractalwrench]

Goal

This makes it possible to override the unhandled property for JVM errors. This allows users to discard certain events from contributing to the stability score.

Changeset

• Renamed HandledState to SeverityReason for greater alignment with notifier spec
• Added setter to Event which allows altering the unhandled state
• If the unhandled has been changed, unhandledOverridden is serialized in the JSON payload
• Updated DeliveryDelegate so that it caches unhandled errors even if a user has set their unhandled state to false

Testing

Updated and added to existing unit test coverage for unhandled state serialization. Added new E2E tests to assert that unhandled can be overwritten both ways, which have been run locally.

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1441.41	1361.3
arm64_v8a	369.25	287.33
armeabi	348.77	266.84
armeabi_v7a	332.38	250.47
x86	410.2	328.28
x86_64	389.72	307.8

Generated by 🚫 Danger

[tomlongridge]

There seem to be a couple of MR changes - is that deliberate? (including some sort of change on features/fixtures/mazerunner/gradlew)

[fractalwrench]

Addressed inline review comments. There are two parts to the mazerunner changes which are intentional:

1. Update Gemfile and Gemfile.lock so that they point at the latest mazerunner when running tests locally
2. Make the gradle wrapper executable through chmod +x which allows for the mazerunner test fixture to be built from the command line