bin: Fix exit code propagation

[tchx84]

Now that we're loading entrypoints as modules, exit codes no longer propagate in the same way. Therefore, we must do that explicitly through the system exit function.

Without this the exit code is always ZERO and in practical terms all CI pipelines depending on this are always passing even when the tests are actually failing.

See 7825a0c.

To reproduce:

1. Build latest jasmine-gjs from master (c6aa28b).

2. Use the following sample test.

describe('test', function() {

    it('fails', function() {
        expect('one').toEqual('two');
    });
});

3. Run it:

$ jasmine --no-config --verbose --no-color sample.js
$ echo $?

With out this PR the exit code is always ZERO.

[tchx84]

@ptomato I addressed your comments, plus removed some unnecessary function wrapping...