Connect loglevel and debug mode

Copy and pasted from !1327 (closed):

I indeed think its useful to have loglevel=debug even without debug=true, since the latter will show error messages to the users and might expose information. So we should add documentation for this. But I still think its a good idea to set loglevel=debug automatically when debug=true, which still allows to set the loglevel however you want if debug mode is turned off.