Make it clear that get_last_modified should not raise an exception, but return None, if it cannot look up a file

parso/cache.py

@@ -82,9 +82,8 @@ def load_module(hashed_grammar, file_io, cache_path=None):
     """
     Returns a module or None, if it fails.
     """
-    try:
-        p_time = file_io.get_last_modified()
-    except FileNotFoundError:
+    p_time = file_io.get_last_modified()
+    if p_time is None:
         return None
 
     try:

parso/file_io.py

@@ -14,10 +14,13 @@ class FileIO(object):
 
     def get_last_modified(self):
         """
-        Returns float - timestamp
-        Might raise FileNotFoundError
+        Returns float - timestamp or None, if path doesn't exist.
         """
-        return os.path.getmtime(self.path)
+        try:
+            return os.path.getmtime(self.path)
+        except OSError:
+            # Might raise FileNotFoundError, OSError for Python 2
+            return None
 
     def __repr__(self):
         return '%s(%s)' % (self.__class__.__name__, self.path)