Add error messages for missing variables when :strict

Hi,

This is a modification to an old pull request which never got merged. https://github.com/Shopify/liquid/pull/104

I felt that the original pull request was too extreme when a variable was missing by throwing an exception. If several variables were missing, then an exception was thrown for the first one but errors did not contain all the missing variables so you'd have to test it multiple times catching each exception.

My modified and simpler version populates the errors with messages for all missing variables and doesn't throw an exception.

Basically, I wanted an easy way to warn users about missing variables when creating of previewing a template.

Regards,
- Daniel