Error message if press "make the magic" button w/o inputting anything in fields

[rlskoeser]

I did this before but then it got lost somehow (?). Found and cherry-picked from the earlier commit.

Needs some refinement. Error message is in smartstash.core.forms; it's displayed on the site index template and almost certainly needs to be styled and positioned.

[moltude]

This has been fixed on dev