Include ImportError information in ImproperlyConfigured error raised in s3boto3.py

[OddBloke]

I'm currently debugging an environment where we're seeing the ImproperlyConfigured error raised when importing the various boto libraries that s3boto3.py needs to function. It would be easier to debug based on the traceback if the ImproperlyConfigured error included some information about the ImportError that caused it.

(As it is now, we have to enter the environment in question and do some manual importing in a Python shell to work out which one it is. Not the worst, but certainly not as good as including it in my logs.)

[OddBloke]

Thanks!