File operations always raise a ENOENT error when a file doesn't exist.
Checking the file exists before the operation adds a race condition
condition where the file could be removed between operations. As the
operation already raises an error on a missing file, avoid this race and
avoid checking the file exists twice. Instead only check a file exists
by catching the ENOENT error.
New Storage.get_{accessed,created,modified}_time() methods convert the
naive time from now-deprecated {accessed,created_modified}_time()
methods into aware objects in UTC if USE_TZ=True.
