Actually return null on error in parseIndentMode

The documentation string says that `parseIndentMode(String string)` should
return `null` to indicate failure, but it actually just threw exceptions
all over the place.

I’ve kept the exceptions around because they might be useful later, to
get a better error message. Maybe there could be a variable top-level
object `lastException` similar to C’s `errno`?
Use test-samples/<filename>.ceylon.options