make ec2 not to bail when boto is not installed

* however make it bail if EC2LatentBuildSlave is used anyway

master/buildbot/buildslave/ec2.py

@@ -25,14 +25,16 @@
 import re
 import time
 
-import boto
-import boto.ec2
-import boto.exception
+try:
+    import boto
+except ImportError:
+    boto = None
 
 from twisted.internet import defer
 from twisted.internet import threads
 from twisted.python import log
 
+from buildbot import config
 from buildbot import interfaces
 from buildbot.buildslave.base import AbstractLatentBuildSlave
 
@@ -62,6 +64,10 @@ def __init__(self, name, password, instance_type, ami=None,
                  placement=None, price_multiplier=1.2, tags={}, retry=1,
                  retry_price_adjustment=1, product_description='Linux/UNIX'):
 
+        if not boto:
+            config.error("The python module 'boto' is needed to use a "
+                         "EC2LatentBuildSlave")
+
         AbstractLatentBuildSlave.__init__(
             self, name, password, max_builds, notify_on_missing,
             missing_timeout, build_wait_timeout, properties, locks)