Update blueprints.rst - Add Error Handlers

"Added information about custom error handlers in Blueprints with example based simple_page.  Includes link to errorpages."

I was looking for a simple explanation of the use Blueprint-specific error handlers.  Couldn't find one in the documentation, but I did figure out how to do it by looking at the actual Blueprint code.  I figured I'd add this to the doc to save others a little time in the future.