pressing `c` in Variables yields an IOError and breaks the variable viewer

``` py
Traceback (most recent call last):
File ".../pudb/var_view.py", line 244, in get_stringifier
execfile(expanduser(iinfo.display_type), custom_stringifier_dict)
IOError: [Errno 2] No such file or directory:
Hit enter:
```

I do this all the time, it's incredibly annoying, since I expect `c` to trigger **Continue**

With some patching, I managed to get something like:

```
does not contain a function named pudb_stringifier atthe module level.
```

I sent #163 to fix the spaces

afaict the display_type is empty, ... when using %s (especially at the beginning of a string), you should special case empty so that the output isn't as incomprehensible as here...

with a bit more cajoling, I got:

```
self: ERROR: Invalid custom stringifier file: pudb_stringifer not defined.
```

As a user, an error like this should result in a dialog, not random text dumped in the ui, and definitely not a fairly persistent error in my Variables window. Certainly not without an explanation of how to undo this.

The obvious way to "recover" is by quitting. If you're doing something reproducible, that might not be a big deal, but if it isn't reproducible, or setup is slow/painful, it's a big deal.