Upgrade the warn for EntryNodes without UseEntryGuards to an error

fixes bug 8180

changes/bug8180

@@ -0,0 +1,7 @@
+  o Minor bugfixes (security usability):
+    - Elevate the severity of the warning message when setting
+      EntryNodes but disabling UseGuardNodes to an error. The outcome
+      of letting Tor procede with those options enabled (which causes
+      EntryNodes to get ignored) is sufficiently different from what
+      was expected that it's best to just refuse to proceed. Fixes bug
+      8180; bugfix on 0.2.3.11-alpha.

src/or/config.c

@@ -1,4 +1,4 @@
-/* Copyright (c) 2001 Matej Pfajfar.
+ /* Copyright (c) 2001 Matej Pfajfar.
  * Copyright (c) 2001-2004, Roger Dingledine.
  * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
  * Copyright (c) 2007-2012, The Tor Project, Inc. */
@@ -3664,9 +3664,9 @@ options_validate(or_options_t *old_options, or_options_t *options,
   if (options->UseBridges && options->EntryNodes)
     REJECT("You cannot set both UseBridges and EntryNodes.");
 
-  if (options->EntryNodes && !options->UseEntryGuards)
-    log_warn(LD_CONFIG, "EntryNodes is set, but UseEntryGuards is disabled. "
-             "EntryNodes will be ignored.");
+  if (options->EntryNodes && !options->UseEntryGuards) {
+    REJECT("If EntryNodes is set, UseEntryGuards must be enabled.");
+  }
 
   options->_AllowInvalid = 0;
   if (options->AllowInvalidNodes) {