Improve error messages.

Improve function name: ensureDefaultType: It runs the check function, so
checkDefault is much easier to understand.

pkgs/lib/options.nix

@@ -66,18 +66,18 @@ rec {
               if all opt.check list then
                 opt.merge list
               else
-                throw "One of the definitions has a bad type.";
+                throw "One of option ${name} values is of a bad type.";
           }
         else opt;
 
-      ensureDefaultType = opt:
+      checkDefault = opt:
         if opt ? check && opt ? default then
           opt // {
             default =
               if opt.check opt.default then
                 opt.default
               else
-                throw "The default value has a bad type.";
+                throw "The default value of option ${name} is of a bad type.";
           }
         else opt;
 
@@ -136,7 +136,7 @@ rec {
 
         # override settings
         ensureMergeInputType
-        ensureDefaultType
+        checkDefault
         handleOptionSets
       ];