Enforce filter format when setting filter

[bkreider]

I accidentally set a filter in the repl, but didn't see the error until later. Disallow setting invalid filters.

[mcg1969]

Thanks for noting this. FYI—the REPL functionality seemed really cool when I included it but honestly I haven't given it much use. Part of the problem is understanding which options should persist between REPL commands and which should not.