Fix bugs about range request and explorer crashes

Lua docs

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.

minor fixes to reduce warnings in mingw

avoid defining macros already present on mingw32

Send http error

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.

update