cleaner message when CLI can't load app

When loading the app fails for the --help command, only the error
message is shown, then the help text. The full traceback is shown for
other exceptions. Also show the message when loading fails while
getting a command, instead of only "command not found". The error
message goes to stderr to match other error behavior, and is in red
with an extra newline to make it more obvious next to the help text.

Also fixes an issue with the test_apps fixture that caused an imported
app to still be importable after the test was over and the path was
reset. Now the module cache is reset as well.

CHANGES.rst

@@ -12,6 +12,8 @@ Unreleased
 -   Passing ``script_info`` to app factory functions is deprecated. This
     was not portable outside the ``flask`` command. Use
     ``click.get_current_context().obj`` if it's needed. :issue:`3552`
+-   The CLI shows better error messages when the app failed to load
+    when looking up commands. :issue:`2741`
 -   Add :meth:`sessions.SessionInterface.get_cookie_name` to allow
     setting the session cookie name dynamically. :pr:`3369`
 -   Add :meth:`Config.from_file` to load config using arbitrary file