Do not fail in last_modified if file is absent

* wcs/qommon/storage.py:
   in last_modified generic implementations do not fail when stat fails
   (usually because the file/directory does not exist), instead return
   current time.

wcs/qommon/storage.py

@@ -1,4 +1,5 @@
 import os
+import time
 import pickle
 
 from quixote import get_publisher
@@ -332,11 +333,21 @@ class StorableObject(object):
 
     def last_modified_id(cls, id):
         filename = os.path.join(cls.get_objects_dir(), fix_key(id))
-        stat = os.stat(filename)
-        return stat.st_mtime
+        mtime = 0
+        try:
+            stat = os.stat(filename)
+            mtime = stat.st_mtime
+        except OSError:
+            mtime = int(time.time())
+        return mtime
     last_modified_id = classmethod(last_modified_id)
 
     def last_modified(cls):
-        stat = os.stat(cls.get_objects_dir())
-        return stat.st_mtime
+        mtime = 0
+        try:
+            stat = os.stat(cls.get_objects_dir())
+            mtime = stat.st_mtime
+        except OSError:
+            mtime = int(time.time())
+        return mtime
     last_modified = classmethod(last_modified)