Deployer raises wrong exception if charm path doesn't exist

Version:
0.6.4-0ubuntu1

Use-case:
Juju-deployer is given an invalid path to a charm.

Experienced behavior:
"TypeError: coercing to Unicode: need string or buffer, NoneType found" is raised from os.path.exists.

This exception is misleading, suggesting a string encoding issue.

Traceback:
http://pastebin.ubuntu.com/24329128/

Expected behavior:
An exception is raised, indicating that the path to the charm does not exist.

Steps to recreate:

Run juju-deployer against any bundle that has an invalid path for "charm"

Example bundle json:
http://pastebin.ubuntu.com/24329128/