feat(nix/yants): improve error message for errornous predicate

while trying to yantsify `mkSecrets` in https://cl.tvl.fyi/c/depot/+/4688,
I(zseri) needed to debug a failing evaluation which boiled down
to a result.ok containing something which wasn't boolean,
but the error message didn't indicate where that value came from.

I debugged yants and found that the only place which didn't
simply combine boolean values or use functions which always
return booleans, I managed to isolate the error to the
`pred v` expression. To avoid the necessity to debug yants
to find this, I improve the error message for this case
to mention that
- a restriction predicate is invalid
- what's the name of the failing restriction
- the unexpected predicate return value

Change-Id: I6c570a33ccc5afc445f208e2e8855c49fb37abaf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4698
Tested-by: BuildkiteCI
Reviewed-by: zseri <zseri.devel@ytrizja.de>
Reviewed-by: tazjin <mail@tazj.in>
Autosubmit: zseri <zseri.devel@ytrizja.de>

nix/yants/default.nix

@@ -317,10 +317,15 @@ in lib.fix (self: {
       in
         if !(t.checkToBool res)
         then res
-        else {
-          ok = pred v;
+        else
+          let
+            iok = pred v;
+          in if isBool iok then {
+            ok = iok;
             err = "${prettyPrint v} does not conform to restriction '${restriction}'";
-        };
+          } else
+            # use throw here to avoid spamming the build log
+            throw "restriction '${restriction}' predicate returned unexpected value '${prettyPrint iok}' instead of boolean";
   };
 
 })