Change error handling for gist tag: raise exception
fix tests for new behaviour
Change error handling in 'post_url' tag: raise exception
raise exceptions in include tag
'gist' tag: switch to ArgumentError exception class

SyntaxError is reserved for Ruby's internal use.

Adjust the tests, including the call to liquid to
make it rethrow ArgumentErrors.
Merge pull request #1514 from maul-esel/tag-errors

Consistent error handling in Liquid tags