KMS generate_data_key (#2071)

* Added KMS.generate_data_key and KMS.generate_date_key_without_plaintext

Increase test coverage to cover Key not found

* Added test for kms.put_key_policy key not found

moto/kms/exceptions.py

@@ -0,0 +1,36 @@
+from __future__ import unicode_literals
+from moto.core.exceptions import JsonRESTError
+
+
+class NotFoundException(JsonRESTError):
+    code = 400
+
+    def __init__(self, message):
+        super(NotFoundException, self).__init__(
+            "NotFoundException", message)
+
+
+class ValidationException(JsonRESTError):
+    code = 400
+
+    def __init__(self, message):
+        super(ValidationException, self).__init__(
+            "ValidationException", message)
+
+
+class AlreadyExistsException(JsonRESTError):
+    code = 400
+
+    def __init__(self, message):
+        super(AlreadyExistsException, self).__init__(
+            "AlreadyExistsException", message)
+
+
+class NotAuthorizedException(JsonRESTError):
+    code = 400
+
+    def __init__(self):
+        super(NotAuthorizedException, self).__init__(
+            "NotAuthorizedException", None)
+
+        self.description = '{"__type":"NotAuthorizedException"}'