Show an error message when translate-shell is missing

[szotsaki]

When someone doesn't install translate-shell (retested by just renaming the executable), krunner-translator shows only an empty "Google Translate" message or when it's not the only one in the result list, just an empty output (even without "Google Translate".

Could you please emit an error message like <Error: translate-shell executable cannot be found> to inform the user why krunner-translator doesn't give back results and to be able to properly install the missing dependency?