Fix python path discovery if not called python

[techalchemy]

Fix python path discovery if not called python

• Begin a refactor of delegator.run invocation to ensure we capture
  and handle failures with our own exception wrappers
• Additoinally capture output and error logging and command information
  when running in verbose mode (should avoid significant repitition in
  the codebase)
• Refactor which and system_which to fallback to pythonfinder's
  implementation
• Abstract is_python_command to identify whether we are looking for
  python, this enables us to rely on pythonfinder.Finder.find_all_python_versions()
  to ensure we aren't skipping python versions
• Should be resilient to environments where there is no which utility
• Fixes pipenv check --system fails #2783
• Fixes 2018.11.26: pipenv emits pythons errors when creating a virtualenv #3363
• Fixes Broken Since Updating to OSX Mojave #3513
• Fixes "Pipenv --three" (or any other) command hangs on "creating virtual environment..." for fresh Virtual Environment #3533