Add mix task to check if the extension install succeeded

Added

• Add a mix task to check the extension install.

  Run mix appsignal.check_install to see if the NIF and agent were successfully installed. If not, it will return with exit code 1. Use this in your CI or build step to check if AppSignal was installed correctly before deploying or starting your application.

Fixed

• Fix an issue where the check-in scheduler would crash when failing to send a check-in due to a network error.

View the Elixir package v2.15.3 changelog for more information.