Prevent Queriers from having a single error output parameter

[jonbodner]

Right now, you can define a querier with 0, 1, or 2 output parameters. An error is only returned if there are two output parameters. If you use 1 output parameter of type error, the error is swallowed. Also, it doesn't make any sense to have a querier that doesn't query; you should return a value.

The fix is to make sure that if there is only a single parameter being returned it cannot be of type error.