Drop the custom load error message in exe/rspec.

There's little need for it nowadays, and it can hide the true error.
Prevent the suite from autorunning once it's been invoked.

This prevents double spec runs when exe/rspec is used to run the suite (i.e. it's run in the regular fashion from the commandline), but there's a 'require "rspec/autorun"' present in spec_helper.rb or elsewhere.
Update some stubs in the runner specs (stale since e76da65).
Warn when an autorun is attempted after having been disabled.