Handles errors occurring during command execution

[pdonadeo]

An error may occur during a command, for example if the user INSERT a NULL value in a non nullable field.

In this case the Postgresql library don't raise any exception: the error is present in the result object, and is accessible via the
#error method.

I changed the error type which is now a "clone" of Postgresql.error with the addition of the new Result_error variant.