MasterAnt Yes, you found a minor bug. This only happens if you have only a single keyword defined, a filter active and decide to delete this only keyword you have while still filtering. It works just fine when there is at least one other keyword defined. You can for example add a keyword for a playlist to fix this issue.
I will add a fix that will reset the filter in this case instead of showing that error message you got.
MasterAnt Is it possible to include keywords in a tagged mp3 file prior to importing into ST3?
No, I am not aware of any standardized keyword tags in mp3 files.