Reference exception in rack.errors
Add test for exception with custom #to_s method
Add custom error message and exception backtrace to `rack.errors`
Simplify building exception message

Also fixes problem with exception without message, in such case class
and message are equal
Pretty print exceptions in rack.errors
Merge branch 'zlw-rack_errors'
Removed Lotus::Action::Throwable#throw in favor of #halt
[ci skip] Changelog
[ci skip] LICENSE.txt -> LICENSE.md
Gem: build only with lib/ and essential files
[ci skip] README