Add useful error response for EXPLAIN on non-supported statements

Technically `EXPLAIN CREATE TABLE` is valid syntax that simply isn't supported yet; SQL error 1064 is not only not useful though it's outright wrong in this scenario. That is fine though an error message should always be helpful and in the case of `EXPLAIN CREATE TABLE` someone had to point out that the syntax was correct though `EXPLAIN` doesn't yet work with `CREATE TABLE`.

I imagine it would be a trivial task for an experienced database developer to trim() the query and see that the first string is `EXPLAIN ` and throw a helpful error message that would save a lot of people a good chunk of time.