Fixed #179: Catch potential FileNotFoundError

When multiple processes share the same file cache, a file can potentially be deleted between the check for existence (`if not os.path.exists(name)`) and the actual read operation.

Author: jaap3

cachecontrol/caches/file_cache.py

@@ -95,12 +95,12 @@ def _fn(self, name):
 
     def get(self, key):
         name = self._fn(key)
-        if not os.path.exists(name):
+        try:
+            with open(name, 'rb') as fh:
+                return fh.read()
+        except FileNotFoundError:
             return None
 
-        with open(name, 'rb') as fh:
-            return fh.read()
-
     def set(self, key, value):
         name = self._fn(key)