Update api.py

Made catch statement error message more explicit for _pack_image

tweepy/api.py

@@ -1271,8 +1271,8 @@ class API(object):
             try:
                 if os.path.getsize(filename) > (max_size * 1024):
                     raise TweepError('File is too big, must be less than %skb.' % max_size)
-            except os.error:
-                raise TweepError('Unable to access file')
+            except os.error as e:
+                raise TweepError('Unable to access file: %s' % e.strerror)
 
             # build the mulitpart-formdata body
             fp = open(filename, 'rb')