init: improve handling of import errors (#840)

* jedi_vim.jedi_import_error: add location

This is useful for debugging.  It contains e.g. the path to parso, if
importing failed from there.

Example:

> Error: jedi-vim failed to initialize Python: jedi#setup_python_imports: could not import jedi: cannot import name 'PythonTokenTypes' (in /…/jedi-vim/pythonx/jedi/jedi/api/completion.py:1). (in function jedi#init_python[3]..<SNR>44_init_python[27]..jedi#setup_python_imports, line 37)

* init: handle jedi_vim.jedi_import_error in Vim plugin

Using `set shortmess+=F` would suppress the `:echom` used in
`jedi_vim.no_jedi_warning` [1].

This patch makes `jedi#setup_python_imports` handle the error instead.

1: https://github.com/neovim/neovim/issues/8675

* Revisit error handling with loading jedi_vim

* jedi#debug_info: display parso submodule separately

* Fix jedi#reinit_python

* fixup! Revisit error handling with loading jedi_vim

* display_debug_info: handle exceptions with environment.get_sys_path

* fixup! Revisit error handling with loading jedi_vim

[ci skip]

plugin/jedi.vim

@@ -48,7 +48,21 @@ endif
 " Pyimport command
 command! -nargs=1 -complete=custom,jedi#py_import_completions Pyimport :call jedi#py_import(<q-args>)
 
-command! -nargs=0 -bar JediDebugInfo call jedi#debug_info()
+function! s:jedi_debug_info()
+    " Ensure the autoload file has been loaded (and ignore any errors, which
+    " will be displayed with the debug info).
+    let unset = {}
+    let saved_squelch_py_warning = get(g:, 'jedi#squelch_py_warning', unset)
+    let g:jedi#squelch_py_warning = 1
+    call jedi#init_python()
+    if saved_squelch_py_warning is unset
+        unlet g:jedi#squelch_py_warning
+    else
+        let g:jedi#squelch_py_warning = saved_squelch_py_warning
+    endif
+    call jedi#debug_info()
+endfunction
+command! -nargs=0 -bar JediDebugInfo call s:jedi_debug_info()
 command! -nargs=0 -bang JediClearCache call jedi#clear_cache(<bang>0)
 
 " vim: set et ts=4: