Added error if chaining update/insert/etc with first()

[wubzz]

Fixes #2061

This throws error in for example

knex(table)
.update({column: value})
.first();

But does not throw for

knex(table)
.first()
.update({column: value});

Since the latter correctly generates an update query.

[elhigu]

It sounds kind of wrong that the latter one is not throwing an error even that .update() removes LIMIT 1 from the query.

Anyways I'm ok with this.