Issue 1548 (sinatra) Describe raise_errors better in README and configuration

[carlwiedemann]

This is sibling work for what was done in https://github.com/sinatra/sinatra/pull/1917

I summarized the changes in a less verbose way in the configuraiton.markdown file.

I believe I have followed the instructions and done the required steps, but please let me know if anything else is necessary. Thanks.