The same error condition is a ValueError in mysqli, be consistent.
Additionally, do not display the argument name for these errors.
As the signatures are overloaded, the argument name may not match
the meaning at all.
The implementation did not check for PQunescapeBytea failure
correctly, because it checked for a null pointer after estrndup,
which certainly cannot happen. Inspection of the PGunescapeBytea
implementation has shown that this function can only fail on OOM,
so let's check for that explicitly and remove false as a possible
return type.
While we're here, avoid an unnecessary copy of the result.
