lib/modules.nix: Add file context to unmerged values in mergeDefinitions

This helps with troubleshooting exceptions in config values, which were hard
to track down for options with many definitions.
The trace will look like:

    error: while evaluating the attribute 'config.foo' at undefined position:
    [...]
    while evaluating the option `foo':
    [...]
    while evaluating definitions from `/home/user/mymod.nix':
    while evaluating 'dischargeProperties' at /home/user/nixpkgs/lib/modules.nix:464:25, called from /home/user/nixpkgs/lib/modules.nix:392:137:
    while evaluating the attribute 'value' at /home/user/nixpkgs/lib/modules.nix:277:44:
    Value error!

where the `/home/user/mymod.nix` module is

    { lib, ... }: {
      options.foo = lib.mkOption {
        type = lib.types.lines;
      };
      config.foo = builtins.throw "Value error!";
    }

lib/modules.nix

@@ -389,7 +389,7 @@ rec {
       let
         # Process mkMerge and mkIf properties.
         defs' = concatMap (m:
-          map (value: { inherit (m) file; inherit value; }) (dischargeProperties m.value)
+          map (value: { inherit (m) file; inherit value; }) (builtins.addErrorContext "while evaluating definitions from `${m.file}':" (dischargeProperties m.value))
         ) defs;
 
         # Process mkOverride properties.