Improve invalid format error in editor

[jum-s]

When entering an invalid value, error is rather scarce:

Some property value format can be confusing, such as P268 (BNF id): is a valid id 13516296h or cb13516296h ?

User experience could be improved by giving some information about what is valid format possibility. This could be done, for example, by pulling value of format as a regular expression on Wikidata property page and display it in the error message.

Another way could be to add a valid placeholder in text field

Related issue: #395