[FLINK-7727] [REST] Improve error logging in StaticFileServerHandlers

[zentol]

What is the purpose of the change

This PR improves the logging of errors in the StateFileServerHandlers

Brief change log

• separate checks for file existence and path not pointing to a directory
• pointing to a directory now returns 405 Method Not Allowed
• pointing outside the root directory now returns 403 Forbidden
• add a debug logging message to each check
• re-order checks, so that we first check whether the path points into the root directory before checking if it exists, to not leak information about resources the user may not access

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

[tillrohrmann]

Closing this PR because it is outdated.