Catch \Throwable instead of \Exception in PHP7

Type errors are not caught by the React Promise implementation in PHP7. It's fairly trivial to implement catch-all for PHP7 and PHP5 without breaking BC.

``` php
try {
  // call a success/error/progress handler
} catch (\Throwable $e) {
  // handle $e
} catch (\Exception $e) {
  // handle $e
}
```
