Improve file not found error

[Mark Keller]

Currently if the external wheel doesn't exist we get a similar error to this:

  File "/home/mkeller/Snowflake/trunk/Python/pvenv_3.6/lib64/python3.6/site-packages/tox_external_wheels/plugin.py", line 55, in tox_package
    return files[0]
IndexError: list index out of range

This should be better