PLAT-5359] Assume default behaviour for UnobservedTaskExceptions when app.config does not exist

[yousif-bugsnag]

Goal

for .NET 4.5 Bugsnag attempts to load the App.config file to read the value of the ThrowUnobservedTaskExceptions element and this crashes with a FileNotFoundException if there is no App.config file. This PR guards against App.config not existing

Design

Make sure that the file exists before loading, and assume the default behaviour (UnobservedTaskExceptions do not terminate the process) if the config file doesn't exist.

Changeset

Added a File.Exists check to UnhandledException#DetermineUnobservedTerminates

Testing

Ran the unit tests and tested manually using the example apps in the repository

[twometresteve]

LGTM