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
- Fixes #2783

Signed-off-by: Dan Ryan <dan@danryan.co>

pipenv/cmdparse.py

@@ -10,7 +10,7 @@ class ScriptEmptyError(ValueError):
 
 
 def _quote_if_contains(value, pattern):
-    if next(re.finditer(pattern, value), None):
+    if next(iter(re.finditer(pattern, value)), None):
         return '"{0}"'.format(re.sub(r'(\\*)"', r'\1\1\\"', value))
     return value