Remove error check after result is obtained

[popeliv]

Fixes #199.

It looks like what was happening here is that the context was getting cancelled while the goroutine execution was between lines 148 and 152.

After the result gets sent on rowsChan, the QueryContext function returns before line 153 is reached and errorChan gets closed.

I don't know how write a test to reproduce this bug, but according to the issue, line 153 was causing the panic so this should fix it.

Apart from that every test still passes.