hsperl_init prints to stderr in event of failure (and exits the process)

[phlummox]

The hsperl_init C function is poorly behaved in the event it fails to parse a given Perl-code "preamble": (a) it prints to stderr, bad behaviour for a library, and (b) calls exit(), terminating the whole process. See here:

    if (SvTRUE(ERRSV)) {
        STRLEN n_a;
        // TODO: see if we can raise a proper exception instead
        fprintf(stderr, "Error init perl: %s\n", SvPV(ERRSV,n_a));
        // NO. Should _NOT_ be aborting, mid-program.
        exit(1);
    }

This should be fixed – we should indicate errors through our return value. Either we can return a NULL pointer on error, and provide an "out" parameter where an error code can be set; or we can have an int return value (0 for success, anything else for failure) and return the Interpreter pointer via an Interpreter **.

The particular condition above is admittedly unlikely to occur – unless the preamble being parsed is deliberately changed, Perl should always be able to parse it – so this is low-ish priority, but no doubt there are other errors in hsperl_init we're not currently checking for.