display the right error when a worker can't be used.

gunicorn/app/base.py

@@ -117,7 +117,12 @@ class Application(object):
                 raise
                     
         self.configure_logging()
-        Arbiter(self).run()
+        try:
+            Arbiter(self).run()
+        except RuntimeError, e:
+            sys.stderr.write("\nError: %s\n\n" % e)
+            sys.stderr.flush()
+            sys.exit(1)
     
     def configure_logging(self):
         """\

gunicorn/workers/geventlet.py

@@ -7,8 +7,10 @@ from __future__ import with_statement
 
 
 import os
-
-import eventlet
+try:
+    import eventlet
+except ImportError:
+    raise RuntimeError("You need eventlet installed to use this worker.")
 from eventlet import hubs
 from eventlet.greenio import GreenSocket
 

gunicorn/workers/ggevent.py

@@ -12,8 +12,10 @@ import sys
 if sys.platform == "darwin":
     os.environ['EVENT_NOKQUEUE'] = "1"
 
-
-import gevent
+try:
+    import gevent
+except ImportError:
+    raise RuntimeError("You need gevent installed to use this worker.")
 from gevent.pool import Pool
 from gevent.server import StreamServer
 from gevent import pywsgi, wsgi

gunicorn/workers/gtornado.py

@@ -6,7 +6,10 @@
 import os
 import sys
 
-import tornado.web
+try:
+    import tornado.web
+except ImportError:
+    raise RuntimeError("You need tornado installed to use this worker.")
 from tornado.httpserver import HTTPServer
 from tornado.ioloop import IOLoop, PeriodicCallback
 from tornado.wsgi import WSGIContainer