I don't think it's right to reuse SPIError for this. SPIError is
clearly meant to signal an error in the SPI calls. Of course, we can't
stop users from raising whatever exception they want, but if we're going
to advertise that users can raise exceptions, then we should create
separate exception classes.
I suppose the proper way to set this up would be to create a base class
like plpy.Error and derive SPIError from that.
