(#6969) String failures differentiate between invalid string/version

This patch does two things:

1. splits out the string lookup functionality from define into a seperate

method []

2. Tries to both load the specific version of a string as well as the

current version so that specific error messages can be returned
differentiating between an invalid version/string.

Reviewed-By: Daniel Pittman <daniel@puppetlabs.com>