global: a naoive complete evaluation will fail, make it fail faster

Hopefully the message will make accidental full evaluations of NixPkgs
(and their inevitable failures) easier to notice and debug.

By suggestion from @grahamc (in his IRC gchristensen form)

pkgs/top-level/all-packages.nix

@@ -55,6 +55,13 @@ with pkgs;
 
   stringsWithDeps = lib.stringsWithDeps;
 
+  ### Evaluating the entire NixPkgs naively will fail, make failure fast
+  AAAAAASomeThingsFailToEvaluate = throw ''
+    Please be informed that this pseudo-package is not the only part of
+    NixPkgs that fails to evaulate. You should not evaluate entire NixPkgs
+    without some special measures to handle failing packages, like those taken
+    by Hydra.
+  '';
 
   ### Nixpkgs maintainer tools