even better IOError for no connection file

IPython/parallel/client/client.py

@@ -47,6 +47,7 b' from .view import DirectView, LoadBalancedView'
 # Decorators for Client methods
 #--------------------------------------------------------------------------
 
+
 @decorator
 def spin_first(f, self, *args, **kwargs):
     """Call spin() to sync state prior to calling the method."""
@@ -58,6 +59,10 b' def spin_first(f, self, *args, **kwargs):'
 # Classes
 #--------------------------------------------------------------------------
 
+_no_connection_file_msg = """
+Failed to connect because no Controller could be found.
+Please double-check your profile and ensure that a cluster is running.
+"""
 
 class ExecuteReply(RichOutput):
     """wrapper for finished Execute results"""
@@ -382,6 +387,11 b' class Client(HasTraits):'
 
         self._setup_profile_dir(self.profile, profile_dir, ipython_dir)
         
+        no_file_msg = '\n'.join([
+        "You have attempted to connect to an IPython Cluster but no Controller could be found.",
+        "Please double-check your configuration and ensure that a cluster is running.",
+        ])
+        
         if self._cd is not None:
             if url_file is None:
                 if not cluster_id:
@@ -389,13 +399,17 b' class Client(HasTraits):'
                 else:
                     client_json = 'ipcontroller-%s-client.json' % cluster_id
                 url_file = pjoin(self._cd.security_dir, client_json)
+                if not os.path.exists(url_file):
+                    msg = '\n'.join([
+                        "Connection file %r not found." % compress_user(url_file),
+                        no_file_msg,
+                    ])
+                    raise IOError(msg)
         if url_file is None:
-            raise ValueError(
-                "I can't find enough information to connect to a hub!"
-                " Please specify at least one of url_file or profile."
-            )
+            raise IOError(no_file_msg)
         
         if not os.path.exists(url_file):
+            # Connection file explicitly specified, but not found
             raise IOError("Connection file %r not found. Is a controller running?" % \
                 compress_user(url_file)
             )