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.
emacs: jump: sort-order bugfix

default-value needs its argument to be quoted.

Slightly strangely default-value of 't or nil is 't or nil
respectively so the code

(default-value notmuch-search-oldest-first)

just gives the current value of notmuch-search-oldest-first rather
than intended default-value of this variable.

The symptom is that if you are in a search buffer and use notmuch jump
to run a saved search which does not have an explicitly set sort order
then the sort order of the saved-search is inherited from the current
search buffer rather than being the default search order.

Thanks to Jani for finding the bug.