making exception error string a proper error class.

[jwoertink]

Fixes #120

I was going to write a spec for it, but the specs in this are sort of complicated, and I wasn't sure how to test that the scalar raises an exception.

[straight-shoota]

How about SELECT LIMIT 0?

[jwoertink]

I tried doing this:

    with_dummy do |db|
      expect_raises DB::NoResultsError do
        db.scalar "SELECT LIMIT 0"
      end
    end

but it never raises an exception.

[jwoertink]

ah, nevermind, I was using the wrong thing.

[jwoertink]

Hey, just wanted to ping this. If anyone has a second to look over it. Thanks!

[straight-shoota]

Thanks @jwoertink