meta-gbmc-staging: entity-manager: gracefully handle read-only filesystem

When the filesystem is mounted as read-only (due to eMMC/flash errors),
any filesystem modification operations (remove, create_directory, copy)
in std::filesystem throw exceptions, which crash the entity-manager daemon.

This patch updates these calls to use the std::error_code overloads,
preventing uncaught filesystem exceptions and allowing the daemon to
continue running (or fail gracefully) rather than entering a crash loop.

no fusion2 link
Google-Bug-Id: 516166428
Tested: Compiled locally
Change-Id: I36efe6a780ed1bb05cb8d2135f4d5b8b246ea69b
Signed-off-by: Tom Tung <tomtung@google.com>