- Added mg.onerror, mg.include, mg.redirect
- mg.onerror, if overridden, lets the user trap syntax errors and
  other loadbuffer errors, and dump stack traces and debug info
  for uncaught runtime errors.
- Log unexpected errors from mg.onerror handlers
- Provide mg.cry so custom handlers can reproduce default behavior
- Call mg.onerror handler before unwinding stack
- If mg.onerror returns non-0, stop loading chunks
- Included pages stop parent page processing if mg.onerror returns non-zero
- Included pages don't sent HTTP errors, they throw lua errors

1. fix a bug that when header contains "Range: 100-", the range request
support will fail
2. (windows) fix a bug that when explorer.exe crashes due to some
reason, the tray icon will disappear.

Allow user to display status code errors

This reverts commit 27fe9c8f.

Allows user code to display its own error status responses or just log
the errors. Is called even in cases where begin_request is not called.