Syntax/type errors should be ephemeral

[byorgey]

Currently, if you run a .sw file that has syntax or type errors, the error messages go into your log, and hence stick around forever. This is not ideal since (1) who cares about past error messages, and (2) when you load a .sw file again it is hard to know whether you have fixed the problems since the error messages are still there in your log.

Somehow the errors should be shown to you but disappear later. Maybe we could make a new kind of "ephemeral" log message that gets deleted at some point, perhaps the next time anything gets added to the log. We could make run generate an ephemeral OK, loaded kind of message as well, so it would replace any errors from the previous run attempt.