Convert ExceptionExtensions to LocalVariables integration.

This prevents binding_of_caller from being loaded unless all of the
following are true:

1. The `send_local_variables` configuration option is enabled.
2. binding_of_caller has been required already (we no longer do this
automatically)

In some cases binding_of_caller was in the Gemfile, but loading it
automatically was undesirable as seen in #88 where it was slowing down
test runs. This leaves everything up to the user.