Refactor init method: add jedi#init_python

- jedi#init_python is used to init the Python environment.
 - jedi#setup_py_version is the single point where PythonJedi is
   defined, and will configure `PythonJedi` to output an error in case
   initialization failed.
 - jedi#force_py_version_switch will throw an error when
   g:jedi#force_py_version is not handled (e.g. invalid or "auto" and
   unresolved).
 - When g:jedi#force_py_version is provided, it will be used always.
 - Use WarningMsg highlight for errors, which are centralized and
   handled through exceptions.

ftplugin/python/jedi.vim

@@ -1,4 +1,4 @@
-if !has('python') && !has('python3')
+if !jedi#init_python()
     finish
 endif
 " ------------------------------------------------------------------------