Improve the error message when a `SITE` is not specified

[ehogan]

Following on from #210, the error message return when running cylc vip is:

% cylc vip
$ cylc validate CMEW
Jinja2Error: Jinja2 Assertion Error: SITE must be set to something other than an empty string
File CMEW/flow.cylc
  #!jinja2
  # flow.cylc
  {{ assert(SITE != "", "SITE must be set to something other than an empty string") }}	<-- Exception

Improve this message by adding something about the fact that the SITE can be set by using -O <site>.