cli: Be more helpful when .notmuch-config does not exist

Previously, if the user ran any subcommand that required a
configuration (e.g., notmuch new) but didn't have a configuration,
notmuch would give the rather un-friendly and un-actionable message

  Error reading configuration file .notmuch-config: No such file or directory

Since this condition is expected for new users, this patch adds
specific handling for the file-not-found case to give a message that
is friendly and actionable.