Antlr Parser Errors

[afscrome]

Need to do some more work around error messages from the parser to ensure they are actionable to fix. So far I've identified the following:

• [x] Unexpected #end should provide a clear error e.g. "Unexpected #end" + line number
• [ ] Missing #end after #if or custom block directive should indicate the name & position of the block directive missing the #end
• [ ] HandleFailures in AntlrVelocityParser should throw a more appropriate exception type.