fix const error responses for Rainbows!

fix const error responses for Rainbows!

Rainbows! relies on the ERROR_XXX_RESPONSE constants of unicorn
4.x.  Changing the constants in unicorn 4.x will break existing
versions of Rainbows!, so remove the dependency on the constants
and generate the error response dynamically.

Unlike Mongrel, unicorn is unlikely to see malicious traffic and
thus unlikely to benefit from making error messages constant.

For unicorn 5.x, we will drop these constants entirely.

(Rainbows! most likely cannot support check_client_connection
 consistently across all concurrency models since some of them
 pessimistically buffer all writes in userspace.  However, the
 extra concurrency of Rainbows! makes it less likely to be
 overloaded than unicorn, so this feature is likely less useful
 for Rainbows!)