Merge pull request #1486 from grahamc/detect-already-running

Test to see if nix-daemon is running already

scripts/install-darwin-multi-user.sh

@@ -296,6 +296,19 @@ EOF
 Nix already appears to be installed, and this tool assumes it is
 _not_ yet installed.
 
+$(uninstall_directions)
+EOF
+    fi
+
+    if pgrep nix-daemon 2> /dev/null >&2; then
+        failure <<EOF
+Nix seems to be partially installed, because the nix-daemon is
+currently running. It may be in a launchd service. Please stop the
+the old nix-daemon, and try again.
+
+If you have an existing launchd plist for nix-daemon, please delete
+it.
+
 $(uninstall_directions)
 EOF
     fi