Check that function loading worked correctly.

In some odd circumstances - typically, just development errors - you can wind
up in a world where a function correctly loads but isn't available.

This ensures that the method is defined as expected and raises a clear, quick
error if not. The alternative is an unclear stack overflow raised because we
recurse forever through method_missing.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>

Conflicts:
lib/puppet/parser/scope.rb