add test for creating already existing keypair

tests/test_ec2/test_key_pairs.py

@@ -1,6 +1,7 @@
 import boto
 import sure  # noqa
 
+from boto.exception import EC2ResponseError
 from moto import mock_ec2
 
 
@@ -15,3 +16,15 @@ def test_key_pairs_create():
     conn = boto.connect_ec2('the_key', 'the_secret')
     kp = conn.create_key_pair('foo')
     assert kp.material.startswith('---- BEGIN RSA PRIVATE KEY ----')
+
+
+@mock_ec2
+def test_key_pairs_create_exist():
+    conn = boto.connect_ec2('the_key', 'the_secret')
+    kp = conn.create_key_pair('foo')
+    assert kp.material.startswith('---- BEGIN RSA PRIVATE KEY ----')
+    # Call get_all_instances with a bad id should raise an error
+    conn.create_key_pair.when.called_with('foo').should.throw(
+        EC2ResponseError,
+        "The keypair 'foo' already exists."
+    )