Fix Storage Class error handling (#1653)

Added check for valid storage classes in set_key and copy_key
added unit test for standard storage and infrequent access

moto/s3/exceptions.py

@@ -168,3 +168,13 @@ class InvalidNotificationEvent(S3ClientError):
             "InvalidArgument",
             "The event is not supported for notifications",
             *args, **kwargs)
+
+
+class InvalidStorageClass(S3ClientError):
+    code = 400
+
+    def __init__(self, *args, **kwargs):
+        super(InvalidStorageClass, self).__init__(
+            "InvalidStorageClass",
+            "The storage class you specified is not valid",
+            *args, **kwargs)