Improve deleting-missing-backup exception handling

When deleting missing backup, OSError could be checked to make
sure that the actual error was that the file does not exist.

Change-Id: I988370d702d87acde534670575d599159b48c58b
Signed-off-by: Xiaojun Liao <xiaojunliao85@gmail.com>

cinder/backup/drivers/posix.py

@@ -17,6 +17,7 @@
 """Implementation of a backup service that uses a posix filesystem as the
    backend."""
 
+import errno
 import os
 import os.path
 import stat
@@ -129,8 +130,10 @@ class PosixBackupDriver(chunkeddriver.ChunkedBackupDriver):
         path = os.path.join(self.backup_path, container, object_name)
         try:
             os.remove(path)
-        except OSError:
-            pass
+        except OSError as e:
+            # ignore exception if path does not exsit
+            if e.errno != errno.ENOENT:
+                raise
 
     def _generate_object_name_prefix(self, backup):
         timestamp = timeutils.utcnow().strftime("%Y%m%d%H%M%S")