Make monkeypatched #exception cause fewer exceptions

The load sequence is such that  `pry-exception_explorer.rb` pulls in `pry-exception_explorer/core_extensions.rb` before `PryExceptionExplorer.enabled?` is defined, so it has the potential to cause infinite recursion.

This patch makes it guarded against that, though what also works is moving the `require 'pry-exception_explorer/core_extensions'` down below the ` module PryExceptionExplorer` definition.