Throw a warning when imports and code are mixed in a cell

[hamelsmu]

Right now we don't have good error messages when imports & code are mixed in a cell and things can hang leaving the user confused. We should catch this and throw a warning

[jph00]

Note: it's fine to mix in an exported cell

[dsm-72]

@jph00 I am not sure if this is true. I have a simple notebook with constants.ipynb and it has basically

#export
import os, inspect
__file = inspect.getfile(lambda: None)
ROOT_DIR = 'here'

running nbdev_preview yields

Preparing to preview
[ 1/13] constants.ipynb
ERROR: No language set for Jupyter notebook