Remove string comparisons for illegal ACL

We should use try/except syntax to try to get rid of
unnecessary string comparisons.

Change-Id: I8dcd902beb27911ea52f8721355a336ccb1b2206

swift3/exception.py

@@ -24,3 +24,7 @@ class NotS3Request(S3Exception):
 
 class BadSwiftRequest(S3Exception):
     pass
+
+
+class ACLError(S3Exception):
+    pass