Adding warning for running from bare cmd
Update links to developer guide in error messages to new location
on the dotnet/corefx wiki.
Cause build.cmd to set errorlevel to 1 on failed prereqs
Cause appveyor to run CI build in Dev12 developer command prompt.
Merge pull request #174 from FiveTimesTheFun/issue-60

Update links in error messages to correct developer documentation on wiki