home-environment: extra message on nix-env error

When profile installation fails during activation we'll print an extra
message that explain that, if the error is due to conflicting
packages, then it may be that the user has a manually installed copy
of the package.

Fixes #1244

modules/home-environment.nix

@@ -493,7 +493,24 @@ in
         ''
       else
         ''
-          $DRY_RUN_CMD nix-env -i ${cfg.path}
+          if ! $DRY_RUN_CMD nix-env -i ${cfg.path} ; then
+            cat <<EOF
+
+          Oops, nix-env failed to install your new Home Manager profile!
+
+          Perhaps there is a conflict with a package that was installed using
+          'nix-env -i'? Try running
+
+              nix-env -q
+
+          and if there is a conflicting package you can remove it with
+
+              nix-env -e {package name}
+
+          Then try activating your Home Manager configuration again.
+          EOF
+            exit 1
+          fi
         ''
     );