pep517.wrappers.BackendUnavailable doesn't actually print out which backend

[ghost]

I just ran into this error in travis:

E               Traceback (most recent call last):
E                 File "<string>", line 9, in <module>
E                 File "/home/travis/build/kivy/python-for-android/.tox/py3/lib/python3.5/site-packages/pythonforandroid/pythonpackage.py", line 444, in _extract_metainfo_files_from_package_unsafe
E                   reqs = hooks.get_requires_for_build_wheel({})
E                 File "/home/travis/build/kivy/python-for-android/.tox/py3/lib/python3.5/site-packages/pep517/wrappers.py", line 71, in get_requires_for_build_wheel
E                   'config_settings': config_settings
E                 File "/home/travis/build/kivy/python-for-android/.tox/py3/lib/python3.5/site-packages/pep517/wrappers.py", line 162, in _call_hook
E                   raise BackendUnavailable
E               pep517.wrappers.BackendUnavailable

But now I feel left guessing. Which backend? I'm guessing setuptools, but it would be extremely helpful if the error could actually print that out

[pfmoore]

Well, whichever backend your project specifies - but it's a fair point, improving the error would be useful.

[ghost]

@pfmoore this is called on some random external project indirectly, you've seen my scanning code 🙂 although my own error before could have told me which one and didn't either, so this is as much my own problem 😬 (which I'm already in the process of fixing) but still, would be useful to have more info here

[pradyunsg]

Related to #10, but worth keeping open separately.

PRs welcome! ^>^