Helpful message when the web server cannot start listening.

src/web-server.coffee

@@ -68,13 +68,38 @@ class $WebServer extends $EventEmitter
 
     ++@_notYetListening
 
-    # Helpful message.
+    errorHandler = (error) ->
+      # `address()` can only be used once listening.
+      address = do ->
+        return socket if socket?
+        "#{host}:#{port}"
+
+      # Prints a (hopefully) helpful message if the server could not
+      # listen.
+      console.log "[WARN] WebServer could not listen on #{address}"
+      switch error.code
+        when 'EACCES'
+          console.log '       Access denied.'
+          if port < 1024
+            console.log '       Ports < 1024 are often reserved to privileges users.'
+        when 'EADDRINUSE'
+          console.log '       Address already in use.'
+
+      # This server will never start listening.
+      --@_notYetListening
+
+    # Registers the error handler.
+    server.on 'error', errorHandler
+
     server.once 'listening', =>
+      # Removes the error handler.
+      server.removeListener 'error', errorHandler
+
+      # Prints a helpful message.
       address = server.address()
       if $_.isObject address
         {address, port} = address
         address = "#{address}:#{port}"
-
       console.log "WebServer listening on #{address}"
 
       # If the web server is listening on all addresses, fire the