app.py: fix ambiguous error message, fixes #322

Current message may be comprehended as "You haven’t specify --repo in
the command-line, please specify it." by translators, and it's wrong.

This commit changes the message to "fatal: \"%s\" is not a directory.
Please specify a correct --repo
\<path\>."

Signed-off-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>