Adding create_secret, exception handle, fix (#1680)

moto/secretsmanager/exceptions.py

@@ -0,0 +1,15 @@
+from __future__ import unicode_literals
+from moto.core.exceptions import JsonRESTError
+
+
+class SecretsManagerClientError(JsonRESTError):
+    code = 400
+
+
+class ResourceNotFoundException(SecretsManagerClientError):
+    def __init__(self):
+        self.code = 404
+        super(ResourceNotFoundException, self).__init__(
+            "ResourceNotFoundException",
+            "Secrets Manager can't find the specified secret"
+        )