Add a cloud watch metric when we fail to start a build via EC2

Fixes #1555

buildman/manager/executor.py

@@ -14,6 +14,7 @@ from functools import partial
 
 from buildman.asyncutil import AsyncWrapper
 from container_cloud_config import CloudConfigContext
+from app import metric_queue
 
 
 logger = logging.getLogger(__name__)
@@ -136,15 +137,20 @@ class EC2Executor(BuilderExecutor):
       )
       interfaces = boto.ec2.networkinterface.NetworkInterfaceCollection(interface)
 
-    reservation = yield From(ec2_conn.run_instances(
-        coreos_ami,
-        instance_type=self.executor_config['EC2_INSTANCE_TYPE'],
-        key_name=self.executor_config.get('EC2_KEY_NAME', None),
-        user_data=user_data,
-        instance_initiated_shutdown_behavior='terminate',
-        block_device_map=block_devices,
-        network_interfaces=interfaces,
-    ))
+    try:
+      reservation = yield From(ec2_conn.run_instances(
+          coreos_ami,
+          instance_type=self.executor_config['EC2_INSTANCE_TYPE'],
+          key_name=self.executor_config.get('EC2_KEY_NAME', None),
+          user_data=user_data,
+          instance_initiated_shutdown_behavior='terminate',
+          block_device_map=block_devices,
+          network_interfaces=interfaces,
+      ))
+    except boto.exception.EC2ResponseError as ec2e:
+      logger.exception('Unable to spawn builder instance')
+      metric_queue.put('EC2BuildStartFailure', 1, unit='Count')
+      raise ec2e
 
     if not reservation.instances:
       raise ExecutorException('Unable to spawn builder instance.')