Catch OSError when trying to get the last_update from git

The "git" executable might not be available (eg. in build environments
where the .git dir is anyway no there because tarballs are used). In
that case, the current code failed with an

OSError: [Errno 2] No such file or directory

exception. This is now catched and the code works even if "git" is not
available.

Change-Id: I10f69d1b2cadced4cd155ef9b7f700a2806f44f9

openstackdocstheme/page_context.py

@@ -32,7 +32,7 @@ def _get_last_updated_file(src_file):
                 '--', src_file,
             ]
         ).decode('utf-8').strip()
-    except subprocess.CalledProcessError as err:
+    except (subprocess.CalledProcessError, OSError) as err:
         LOG.info(
             '[openstackdocstheme] Could not get modification time of %s: %s',
             src_file, err)