switch to using ex-info for exceptions

[yogthos]

Currently, the parser throws generic Exception on error, it would be better to use ex-info and set a specific type. This would allow making middleware for printing out friendly errors instead of the generic stack trace.

[djpowell]

It would also be good if we could set the exception cause to the underlying exception here: https://github.com/yogthos/Selmer/blob/master/src/selmer/filter_parser.clj#L120