lib/modules: fix error-message when declaring an option inside `config'

The message I originally implemented here was to catch a mixup of
`config' and `options' in a `types.submodule'[1]. However it looks
rather weird for a wrongly declared top-level option.

So I decided to throw

    error: The option `foo' does not exist. Definition values:
           - In `<unknown-file>':
               {
                 bar = {
                   _type = "option";
                   type = {
                     _type = "option-type";
               ...

           It seems as you're trying to declare an option by placing it into `config' rather than `options'!

for an expression like

    with import ./lib;

    evalModules {
      modules = [
        {
          foo.bar = mkOption {
            type = types.str;
          };
        }
      ];
    }

[1]  fa30c9abed

lib/modules.nix

@@ -162,13 +162,24 @@ rec {
             baseMsg = "The option `${showOption (prefix ++ firstDef.prefix)}' does not exist. Definition values:${showDefs [ firstDef ]}";
           in
             if attrNames options == [ "_module" ]
-              then throw ''
-                ${baseMsg}
-
-                However there are no options defined in `${showOption prefix}'. Are you sure you've
-                declared your options properly? This can happen if you e.g. declared your options in `types.submodule'
-                under `config' rather than `options'.
-              ''
+              then
+                let
+                  optionName = showOption prefix;
+                in
+                  if optionName == ""
+                    then throw ''
+                      ${baseMsg}
+
+                      It seems as you're trying to declare an option by placing it into `config' rather than `options'!
+                    ''
+                  else
+                    throw ''
+                      ${baseMsg}
+
+                      However there are no options defined in `${showOption prefix}'. Are you sure you've
+                      declared your options properly? This can happen if you e.g. declared your options in `types.submodule'
+                      under `config' rather than `options'.
+                    ''
             else throw baseMsg
         else null;