Improve error message when all permitted Exits are down

The old "No specified non-excluded exit routers seem to be running"
message was somewhat confusing.

Fix for 7890.

changes/bug7890

@@ -0,0 +1,4 @@
+  o Minor bugfixes (logging):
+    - Use a more comprehensible log message when telling the user they've
+      excluded every running exit node. Fixes bug 7890; bugfix on
+      0.2.2.25-alpha.

src/or/circuitbuild.c

@@ -1956,9 +1956,10 @@ choose_good_exit_server_general(int need_uptime, int need_capacity)
   }
   if (options->ExitNodes) {
     log_warn(LD_CIRC,
-             "No specified %sexit routers seem to be running: "
+             "No exits in ExitNodes%s seem to be running: "
              "can't choose an exit.",
-             options->ExcludeExitNodesUnion_ ? "non-excluded " : "");
+             options->ExcludeExitNodesUnion_ ?
+             ", except possibly those excluded by your configuration, " : "");
   }
   return NULL;
 }