Errors are suppressed in generator comprehensions

```
>>> list(i for x in range(10))
[]
```

Which should have output

```
>>> list(i for x in range(10))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <genexpr>
NameError: name 'i' is not defined
```