Show precise error messages in option merge failures

For instance, if time.timeZone is defined multiple times, you now get
the error message:

  error: user-thrown exception: The unique option `time.timeZone' is defined multiple times, in `/etc/nixos/configurations/misc/eelco/x11vnc.nix' and `/etc/nixos/configuration.nix'.

while previously you got:

  error: user-thrown exception: Multiple definitions of string. Only one is allowed for this option.

and only an inspection of the stack trace gave a clue as to what
option caused the problem.

nixos/modules/config/shells-environment.nix

@@ -25,11 +25,14 @@ in
       '';
       type = types.attrsOf (mkOptionType {
         name = "a string or a list of strings";
-        merge = xs:
+        merge = args: xs:
           let xs' = filterOverrides xs; in
           if isList (head xs') then concatLists xs'
-          else if builtins.lessThan 1 (length xs') then abort "variable in ‘environment.variables’ has multiple values"
-          else if !builtins.isString (head xs') then abort "variable in ‘environment.variables’ does not have a string value"
+          else if builtins.lessThan 1 (length xs') then
+            # Don't show location info here, since it's too general.
+            throw "The option `${showOption args.prefix}' is defined multiple times."
+          else if !builtins.isString (head xs') then
+            throw "The option `${showOption args.prefix}' does not have a string value."
           else head xs';
       });
       apply = mapAttrs (n: v: if isList v then concatStringsSep ":" v else v);