Fulltext search should be completly deactivated in kiwix-serve if ZIM don´t have a corresponding index

[kelson42]

If I open a ZIM file without fulltext search index and then type in the search box something and type ENTER, then we see something like this

This is very frustrating because as a user I didn´t do something wrong but still get an error.

Kiwix server should not allow to run a fulltext search (and generate a systematic error) if there is not fulltext search index.

What should happen when the user press ENTER is unclear to me at this stage.