The 550 is overused as a return code for multiple error case, e.g.
file not found and/or insufficient permissions to access the file.
So we cannot fail hard in this case.
Adjust test 511 since we now fail later.
Add new test 3027 which check that when MDTM failed, but the file could
actually be retrieved, that in this case no filetime is provided.
Reported-by: Michael Heimpold
Fixes#9357Closes#9387
