Unhelpful error message when trying to run shell that can't be imported

[Peter Inglesby]

When you try to run a specific shell, and that shell can't be imported, you get a traceback with an unhelpful error message:

$ ./manage.py shell -i bpython
Traceback (most recent call last):
  File "./manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/inglesp/.pyenv/versions/inglesham-gallery/lib/python3.6/site-packages/django/core/management/__init__.py", line 367, in execute_from_command_line
    utility.execute()
  File "/Users/inglesp/.pyenv/versions/inglesham-gallery/lib/python3.6/site-packages/django/core/management/__init__.py", line 359, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/inglesp/.pyenv/versions/inglesham-gallery/lib/python3.6/site-packages/django/core/management/base.py", line 294, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/inglesp/.pyenv/versions/inglesham-gallery/lib/python3.6/site-packages/django/core/management/base.py", line 345, in execute
    output = self.handle(*args, **options)
  File "/Users/inglesp/.pyenv/versions/inglesham-gallery/lib/python3.6/site-packages/django/core/management/commands/shell.py", line 124, in handle
    raise ImportError("Couldn't load any of the specified interfaces.")
ImportError: Couldn't load any of the specified interfaces.

Instead you should get CommandError explaining what's gone wrong.

I have a PR which comes from my first attempt at fixing #27670.

[Tim Graham <timograham@…>]

In 0b2e5da6:

Fixed #27721 -- Added interface name to shell's IPython/bython import error.