improve return value convention for hsperl_return_conv

[phlummox]

The C function SV ** hsperl_return_conv (int count) has a needlessly complex way of returning values and errors – it allocates an array of SVs, with up to two NULL terminators (explanation here),

It'd be better to just provide an "out" parameter, that might contain return values or errors, and use an int return value to indicate which is the case. But also - there doesn't seem to be any reason to provide multiple error values. (I think one is intended to be always a string, the other, potentially, a reference to a more complex Perl structure.) We can just provide one, and let the caller work out how best to deal with it.