libs: made error message clearer and more consistent

lib/utils.sh

@@ -21,6 +21,11 @@ export -f stdtime
 # ------------------------------------------------------------------------------
 # Check if a function exists, return 0 if so
 function_exists() {
+    if [[ $# -ne 1 ]]; then
+        prnt E "function_exists(): A function name is required!"
+        die 11 --force
+    fi
+
     LC_ALL=C type -t $1 | grep function
     return $?
 }