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.
253570784c