prevent automatic loading error for missing const

If there is a controller file that we are automatically loading
based on it's filename, but the controller doesn't define the constant
we think it should be, i.e.

``ruby
class NotWhatISaidIWasController < Unveil::Controller
end
``

Then we would cause a ```stack level too deep``` error, when ruby
tries to continuously loop through getting the constant, calling
const_missing, then trying to get the constant again because it's
been overridden, in a loop until the interpreter crashes out.