Handle exception if it raises inside view/models etc. #57

## Remove duplicated code
I think that `__import__(...)` is duplicated code. 
I make a function then use it. 
The role of function is exception handling and importing.

But, I can't judge which style is more better. Which code do you prefer?

* `self._import_app_module("{}.admin".format(app))`
* `self._import_app_module("{}.admin", app)`

## Use traceback
Showing exception manually. 
Maybe extending this code, we can implement good exception handling.