Provide friendlier error for missing setup.py.

[jchristgit]

As suggested in #331, this PR makes tox output a friendlier exception when setup.py could not be found. When this is the case, the following is outputted instead of throwing a MissingFile exception:

Checklist:

• wrote descriptive pull request text
• added/updated test(s)
• updated/extended the documentation
  Should this be documented and if yes, where?
• added relevant issue keyword
  in message body
• added news fragment in changelog folder, see this
• added yourself to CONTRIBUTORS (preserving alphabetical order)

[codecov]

Codecov Report

Merging #650 into master will decrease coverage by 3.12%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## master #650 +/- ## ========================================== - Coverage 93.86% 90.73% -3.13%  ========================================== Files 11 11 Lines 2364 2365 +1 Branches 0 395 +395 ========================================== - Hits 2219 2146 -73  + Misses 145 144 -1  - Partials 0 75 +75

Impacted Files	Coverage Δ
tox/session.py	91.98% <100%> (-2.36%)	⬇️
tox/_verlib.py	71.57% <0%> (-9.48%)	⬇️
tox/_quickstart.py	77.62% <0%> (-5.6%)	⬇️
tox/interpreters.py	77.34% <0%> (-4.69%)	⬇️
tox/_pytestplugin.py	89.95% <0%> (-4.02%)	⬇️
tox/config.py	95.03% <0%> (-2.55%)	⬇️
tox/venv.py	93.76% <0%> (-2.5%)	⬇️

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[asottile]

Here's the answer to "why does it exit 0": https://github.com/Volcyy/tox/blob/d5a68ad9b6313d9505f9ecfdcd32556e040d4e20/tox/_pytestplugin.py#L309

initproj sets up a setup.py by default, you'll probably need to explicitly delete it in your test to demonstrate the behaviour

[jchristgit]

Thanks for telling me that, I added the test case, it removes the file now. There seems to be some issue with one of the Travis builds though, and I'm not sure how to fix that.

[asottile]

I nudged travis-ci for ya :)

This looks good to me, curious to hear one of the more seasoned maintainers chime in on whether we're ok with switching the exception here.