async error handling

Hi,

I just noticed something fairly embarassing about the async code error 
handling, namely it's lack.

Executing an invalid query with an async cursor will not raise any 
exceptions, because the code happily ignores the need to look at 
PQresultStatus. Of course that's entirely my fault, the sync codepath 
does the right thing.

I'll see about fixing it.

Cheers,
Jan