Better warning when dipy is installed without fury

[arokem]

Description

If dipy is installed without fury, and the following line (from our examples) is run:

from dipy.viz import actor, window, ui

the user will likely see an uninformative error:

---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-5eadb5980917> in <module>
----> 1 from dipy.viz import actor, window, ui

ImportError: cannot import name 'actor' from 'dipy.viz' (/Users/arokem/.virtualenvs/test_dipy_no_fury/lib/python3.7/site-packages/dipy/viz/__init__.py)`

This is because the current implementation of optional_pkg doesn't handle from foo import bar imports elegantly.

I think that we should provide more useful information here

[skoudoro]

Fixed by #1977 , closing