Show a better error when running jpm from the wrong directory

[kumar303]

jpm should show a better error when not running from the source of an add-on. Instead it shows something like this:

$ jpm xpi
JPM [info] Starting jpm xpi on undefined
JPM [error] XPI creation failed: Error: Could not parse `package.json` file.

ENOENT: no such file or directory, open '/Users/kumar/package.json'