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

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 `.

Start by searching the workflow's flow.cylc file for the assertion containing “SITE must be set to something other than an empty string,” then inspect how cylc vip reports that error. Update the message to mention setting SITE with -O , and run cylc vip without a SITE to verify the guidance appears.

Written by the indexing model from the issue text.