Increased logging detail/verbosity for epoxy config errors.

Often when there is a syntax error in your epoxy config file you will see a stack trace like so:

```
Traceback (most recent call last):
  File "/home/swstack/projects/epoxy/epoxy/test/test_core.py", line 125, in test_invalid_class
    self.mgr.launch_subgraph(configuration, 'd:main')
  File "/home/swstack/projects/epoxy/epoxy/core.py", line 264, in launch_subgraph
    component = component_reference.get_instance(self.graph)
  File "/home/swstack/projects/epoxy/epoxy/core.py", line 70, in get_instance
    class_ref = getattr(module, class_name)
AttributeError: 'module' object has no attribute 'InvalidComponent'
```

This traceback is not as helpful as it could be since we don't know which `module` it is referring to.  This pull request introduces some added logging and tests to cover these cases.

Epoxy supports different types of configuration mechanisms. I think YAML should be removed and cfg should be changed to config.