Smiley silently quits on unhanded exception

For example, with the following in test.py:

```
#!/path/to/python
print "foo"
raise Exception("bye")
print "bar"
```

```
$ smiley run test.py
foo
$ echo $?
0
```

It's debatable whether or not exiting with zero status is the best thing to do if a script has failed.
