lib/generators: fix toPretty throwing on (partially applied) builtins

An high level example case of this problem occuring can be found below:

nix-repl> lib.generators.toPretty {} (lib.concatStringsSep "\n")
error: 'functionArgs' requires a function, at /home/lukas/src/nix/nixpkgs/lib/trivial.nix:334:42

However this does not happen on other partially applied functions:

nix-repl> lib.generators.toPretty {} (lib.concatMapStringsSep "\n")
"<function>"

The issue, as it turns out is that while builtins are functions,
builtins.functionArgs throws if is passed a builtin or a partially
applied builtin:

nix-repl> lib.generators.toPretty {} builtins.toString
error: 'functionArgs' requires a function, at /home/lukas/src/nix/nixpkgs/lib/trivial.nix:334:42

nix-repl> lib.generators.toPretty {} (builtins.foldl' (a: b: a + b))
error: 'functionArgs' requires a function, at /home/lukas/src/nix/nixpkgs/lib/trivial.nix:334:42

I'm pretty sure this qualifies as a nix bug and should be filed
accordingly, but we can work around it in lib.generators.toPretty by
using tryEval and falling back to {} which functionArgs _should_ return
for builtins.

The nix behavior is inconsistent to say the least:

nix-repl> builtins.functionArgs builtins.functionArgs
error: 'functionArgs' requires a function, at (string):1:1

nix-repl> builtins.typeOf builtins.functionArgs
"lambda"

builtins.functionArgs (a: 1 + a)
{ }

nix-repl> builtins.typeOf (a: 1 + a)
"lambda"

lib/generators.nix

@@ -236,11 +236,16 @@ rec {
         + libStr.concatMapStringsSep introSpace (go (indent + "  ")) v
         + outroSpace + "]"
     else if isFunction v then
-      let fna = lib.functionArgs v;
+      # functionArgs throws in case of (partially applied) builtins
+      # on nix before commit b2748c6e99239ff6803ba0da76c362790c8be192
+      # which includes current nix stable
+      # TODO remove tryEval workaround when the issue is resolved on nix stable
+      let fna = builtins.tryEval (lib.functionArgs v);
           showFnas = concatStringsSep ", " (libAttr.mapAttrsToList
                        (name: hasDefVal: if hasDefVal then name + "?" else name)
-                       fna);
-      in if fna == {}    then "<function>"
+                       fna.value);
+      in if !fna.success || fna.value == {}
+         then "<function>"
          else "<function, args: {${showFnas}}>"
     else if isAttrs    v then
       # apply pretty values if allowed