throw a real error if emacs version is too old

Many people (including me) were confused when trying to install Spacemacs on an
outdated Emacs. A message was issued, but honestly nobody thinks of going to see
the *Messages* buffer when a perfeclty normal vanilla Emacs starts up,
especially not beginners.

This way, an error is visible and users can take corrective actions.

init.el

@@ -20,9 +20,9 @@
 (defconst spacemacs-emacs-min-version   "24.4" "Minimal version of Emacs.")
 
 (if (not (version<= spacemacs-emacs-min-version emacs-version))
-    (message (concat "Your version of Emacs (%s) is too old. "
-                     "Spacemacs requires Emacs version %s or above.")
-             emacs-version spacemacs-emacs-min-version)
+    (error (concat "Your version of Emacs (%s) is too old. "
+                   "Spacemacs requires Emacs version %s or above.")
+           emacs-version spacemacs-emacs-min-version)
   (load-file (concat (file-name-directory load-file-name)
                      "core/core-load-paths.el"))
   (require 'core-spacemacs)