Update launcher scripts with file check, docs

- Add a check that the relevant JS file exists in the expected location,
  before attempting to execute it. Display a message to stderr, if it's
  not found.
- Add a comment above the code, detailing how to symlink the script
  into a directory on the $PATH.

src/gsconnect-preferences.sh

@@ -4,9 +4,21 @@
 #
 # SPDX-License-Identifier: GPL-2.0-or-later
 
+# To use this script as a `gsconnect-preferences` command from any
+# directory (e.g. $HOME/.local/bin, /usr/local/bin/, or somewhere else
+# on your $PATH), you can SYMLINK (do not copy!) it into that location.
+#
+# e.g:
+# D="$HOME/.local/share/gnome-shell/extensions/gsconect@anyholmes.github.io"
+# ln -s $D/gsconnect-preferences.sh $HOME/.local/bin/gsconnect-preferences
+
 case "$0" in
     */*) EXTENSION_DIR=$(dirname $(realpath "$0")) ;;
     *) EXTENSION_DIR=$(dirname $(realpath $(which $0))) ;;
 esac
+if [ ! -f "${EXTENSION_DIR}/preferences-app.js" ]; then
+    >&2 echo "Cannot find preferences-app.js relative to script location!"
+    >&2 echo "Perhaps you copied the script instead of symlinking it?"
+    exit -1
+fi
 exec /bin/env gjs -m "${EXTENSION_DIR}/preferences-app.js" "$@"
-

src/gsconnect.sh

@@ -4,9 +4,21 @@
 #
 # SPDX-License-Identifier: GPL-2.0-or-later
 
+# To use this script as a `gsconnect` command from any directory
+# (e.g. $HOME/.local/bin, /usr/local/bin/, or somewhere else on your
+# $PATH), you can SYMLINK (do not copy!) it into that location.
+#
+# e.g:
+# D="$HOME/.local/share/gnome-shell/extensions/gsconect@anyholmes.github.io"
+# ln -s $D/gsconnect.sh $HOME/.local/bin/gsconnect
+
 case "$0" in
     */*) EXTENSION_DIR=$(dirname $(realpath "$0")) ;;
     *) EXTENSION_DIR=$(dirname $(realpath $(which $0))) ;;
 esac
+if [ ! -f "${EXTENSION_DIR}/service/daemon.js" ]; then
+    >&2 echo "Cannot find service/daemon.js relative to script location!"
+    >&2 echo "Perhaps you copied the script instead of symlinking it?"
+    exit -1
+fi
 exec /bin/env gjs -m "${EXTENSION_DIR}/service/daemon.js" "$@"
-