notmuch emacs mode could be friendlier when the user has never run "notmuch setup"

[Daniel Kahn Gillmor]

in testing out notmuch on a new user account, i just noticed that
bringing up notmuch mode in emacs chokes with an unhelpful error for
users who have never run "notmuch setup".  it seems like there ought to
be a way for the emacs mode to detect this error state and prompt the
user to walk through "notmuch setup" directly from within emacs.

my emacs-fu (and my time) is too limited for me to try to implement this
bit of user-friendliness, but i just wanted to put the idea out there
for folks who might want to take it on as a well-scoped project.

At the very least, the error message could say "please run 'notmuch
setup' from a shell".

The current behavior (notmuch and notmuch-emacs 0.16-1 on debian) is to
open a *notmuch-hello* buffer that says :

  Welcome to notmuch. You have

And in the minibuffer, a message says "notmuch exited with status 1"

*Messages* shows:

process-lines: notmuch exited with status 1

	--dkg

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