Output with missing setup.py could be nicer

[pytoxbot]

• Bitbucket: https://bitbucket.org/hpk42/tox/issue/331
• Originally reported by: @The-Compiler
• Originally created at: 2016-05-13T11:16:35.722

When there's no setup.py, we get:

$ tox 
Traceback (most recent call last):
  File "./.venv/bin/tox", line 11, in <module>
    sys.exit(cmdline())
  File "/home/florian/tmp/.venv/lib/python3.4/site-packages/tox/session.py", line 39, in main
    retcode = Session(config).runcommand()
  File "/home/florian/tmp/.venv/lib/python3.4/site-packages/tox/session.py", line 375, in runcommand
    return self.subcommand_test()
  File "/home/florian/tmp/.venv/lib/python3.4/site-packages/tox/session.py", line 520, in subcommand_test
    path = self.get_installpkg_path()
  File "/home/florian/tmp/.venv/lib/python3.4/site-packages/tox/session.py", line 496, in get_installpkg_path
    path = self._makesdist()
  File "/home/florian/tmp/.venv/lib/python3.4/site-packages/tox/session.py", line 390, in _makesdist
    raise tox.exception.MissingFile(setup)
tox.MissingFile: MissingFile: /home/florian/tmp/setup.py

instead the output could be nicer, like if the setup.py is broken:

$ tox                               
GLOB sdist-make: /home/florian/tmp/setup.py
ERROR: No dist directory found. Please check setup.py, e.g with:
     python setup.py sdist