Consider adding `error` expression type for better error cascading

[christopherswenson]

If you have:

aggregate: 
  x is count()
  y is x * 2

You get two errors:

• "x is not defined"
• "cannot use a scalar value in an aggregate"

The first error is correct. The second error is confusing, and happens because the result of looking up x defaults to a scalar expression type, which then fails the expression type check of aggregate. We could just not emit that error if the data type is error, but that would remove some helpful instances of the error message, e.g. with aggregate: y is scalar_sql_native_typed_thing + 1 (where the data type is error but the expression type is known to be scalar).

In general, we are also not super careful of setting the expressionType correctly when we use errorFor or loggedErrorExpr; in many cases we know something to be an aggregate, but we generate an error/scalar when there are issues.