More clear error on unsupported IN using subqueries instead of expressions

[campoy]

When running this request, I get a very unclear error. What is not supported exactly?

SELECT *
FROM trees
WHERE tree_hash IN (
  SELECT tree_hash
  FROM commits
  WHERE commit_hash = '5e5f7e53896a171959c34483a60db5770a8abac0'
)

The error message doesn't necessarily help explain why this is not supported.

Query Failed - unknown error: unsupported syntax: &sqlparser.Subquery{Select:(*sqlparser.Select)(0xc000a5e340)}

[kuba--]

First of all the query should be (tree_entries instead of trees):

SELECT *
FROM tree_entries
WHERE tree_hash IN (  SELECT tree_hash  FROM commits  WHERE commit_hash = '5e5f7e53896a171959c34483a60db5770a8abac0');

So far we don't support subqueries for conditions/filters (it will require extra work for mysql - here is an open issue: src-d/go-mysql-server#59)

This PR just adds an explicit error message: src-d/go-mysql-server#552