warning 307

I believe we need to learn to write here more about changes we introduce.

Today I have remembered about one of them, new warning number 307.

When CASE statement sequence does not end with ELSE, and neither of possibilities is true, then trap happens. Before this caused a lot of time and efforts to figure out why this or that code crashes. Thus the warning have been added. (:

Upd. By the way, I have contributed the same change (and it got accepted) to the Linux Oberon Revival project, because Vishap is also based on OP2 compiler.