WLS: added initial thoughts for error message specifications.
WLS: added an initial chapter on error messages.
The purpose is to give each error message a code, and to discuss what it means.  Furthermore, some discussion of how to resolve the error and/or how suggestions about the error could be included.
WLS: culled out some error messages which I don't believe are valid.
This has narrowed down the number of error messages to something more acceptable.
Merge branch 'master' of github.com:Whiley/WhileyDocs
GSW: Added examples from typing section.
Not all of these compile yet, and I need to figure out a mechanism for
managing dependencies somehow.