Fix for bad_shell_exec failing depending on the user's shell

Summary: Different shells have different error messages, which trips up
this test case. Switches the test case to a regex covering the
two cases I've seen.

Eg:
```
~/hhvm(branch:master) » echo $SHELL
/usr/bin/zsh

~/hhvm(branch:master) » hphp/hhvm/hhvm hphp/test/run hphp/test/slow/ext_process/bad_shell_exec.php
Running 1 tests in 1 threads

FAILED
1 tests failed
(╯°□°）╯︵ ┻━┻
: hphp/test/slow/ext_process/bad_shell_exec.php
002+ sh: 1: /invalid_path/: not found
002- sh: /invalid_path/: No such file or directory
```

Closes https://github.com/facebook/hhvm/pull/3103

Reviewed By: ptarjan, joelm, fredemmott

Differential Revision: D1435381

Pulled By: svcscm
Define Nast.FGenerator

Summary: Determining if something is a generator is a PITA (we have to
traverse the body), but we're already doing it, in two different places
no less, so let's bite the bullet and flatten that down during naming
into Nast.FGenerator.

Reviewed By: @elgenie

Differential Revision: D1463077