#213 make use: output warnings on console, return non-zero status

[Robert Wenner]

When using -ShowMissingParams / Remarks / Returns / Summaries / Values, please write messages to stderr instead of in the documentation and return non-zero.

To find these problems I now have to go through ALL my generated documentation manually and look for these errors. If one slips by, readers of the documentation think I work sloppy. (This is really bad when using NDoc for published APIs.)

If you return non-zero to the OS, my makefile would stop with an error here, making it impossible to not catch these problems.