improve readability of validator warnings

[Klumbumbus]

example of a validator warning:
tourism=information without information

it would be nice if the keys and tags which are included by {x} are written differently visualized (e.g. italic, other font, other color or highlighting). This way you can better understand which word is a key/tag and which is just a "normal" word of the validator message.

A few validator messages use additional ' characters but this is not optimal and adding this to the other messages would trigger a retranslation of all those messages.