Make dotify-action error out when given invalid actions

src/lib/dotify-action.sh

@@ -7,5 +7,11 @@ dotify-action() {
     action="$DOTIFY_OPT_DEFAULT_ACTION"
   fi
 
+  ! valid_action="$(command -v "dotify-action-${action}")"
+  if [ -z "$valid_action" ]; then
+    echo "ERROR: \"$action\" is not a valid action." >&2
+    return 1
+  fi
+
   dotify-action-${action} "$source" "$target"
 }