Show a usage hint for CustomScriptPlugin in configuration UI

When a formatter tool referenced in CustomScriptPlugin's style cannot be
found (not installed), error messages are easy to miss. They are written
not into systemd journal, but into ~/.xsession-errors, because the error
occurs in the shell rather than in KDevelop directly.

The usage hint that emphasizes the external tool executable dependency
of each custom script style should reduce usage mistakes and wrong
assumptions by users.