Improve error message from parser errors to include more context

This is a request gathered during Couchbase Connect SV 2018.

Currently when a parser error occurs, we give the token right before the error. The complain is for a large query, it may not be easy to derive the place where the error occurred from the token given. As an example, if the parser gives a comma as the token, it's impossible to pinpoint where the comma might be if the query contains multiple commas. As another example, a query may contain many SUM() aggregates and the error token "sum" is given, then it's hard to pinpoint which sum the error is referring to.

If we can give more context around where the error occurs it would be much more helpful in trying to pinpoint the point of error.

E.g., a line number + an offset is a good start, although that still requires manual work to count.

If we can give the query text around the error, together with an error token, that will be more helpful.