add error handling for webdav mkcol according to RFC 4918

[Kuingsmile]

Refer to #5491 使用litmus进行检测，当前版本结果:

参考RFC 4918

1. 根据RFC 4918 9.3.1: 405 (Method Not Allowed) - MKCOL can only be executed on an unmapped URL
2. 根据RFC 4918 9.3.1: 409 (Conflict) The server MUST NOT create those intermediate collections automatically

根据如上两条，添加了对应的error handling代码，修改后检测结果如下：

另外，修改了webdav.go中handleLock函数的一段错误处理代码，原为:

reqPath, status, err := h.stripPrefix(r.URL.Path)
reqPath, err = user.JoinPath(reqPath)
if err != nil {
   return 403, err
}
if err != nil {
  return status, err
}

第二个if判断永远不会有机会执行到，修改为:

reqPath, status, err := h.stripPrefix(r.URL.Path)
if err != nil {
	return status, err
}
reqPath, err = user.JoinPath(reqPath)
if err != nil {
	return 403, err
}