mbed: config: print detailed help in subcommand

Print detailed usage when unsupported arguments are supplied
with mbed config subcommand

mbed/mbed.py

@@ -3249,9 +3249,13 @@ def config_(var=None, value=None, global_cfg=False, unset=False, list_config=Fal
                     action('%s now set as default %s in program "%s"' % (value, name, program.name))
                 else:
                     value = program.get_cfg(var)
-                    action(('%s' % value) if value else 'No default %s set in program "%s"' % (name, program.name))
+                    if value:
+                        action(('%s' % value)
+                    else
+                        action('No default %s set in program "%s"' % (name, program.name))
+                        error("run with -h for detailed usage help")
     else:
-        subcommands['config'].error("too few arguments")
+        error("Too few arguments. Run with -h for detailed help")
 
 
 # Build system and exporters