scripts/oe-package-browser: Handle no packages being built

Give the user a proper error message if there aren't packages built,
rather than a less friendly traceback.

[YOCTO #14619]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b14c176b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>