[scriptassist.pl] "/unknowncommand/ foobar" results in error diagnostic, but message is still sent

## `/unknowncommand/ foobar`

1. send a message with the text `/unknowncommand/ foobar` to some recepient
2. scriptassist displays a diagnostic saying `no script provides '/unknowncommand/'`
3. message is still being sent (`PRIVMSG #foobar :/unknowncommand/ foobar`)

`(2)` does not play too well with `(3)` as the error diagnostic makes one think it _should_ behave like the following:

## `/unknowncommand foobar`

1. send a messaage with the text `/unknowncommand foobar`
2. irssi states `Unknown command: unknown command`
3. scriptassist states `No script provides /unknowncommand`
4. message is *not* being sent.

## Elaboration

It is somewhat misleading that the diagnostic is printed as if `/unknowncommand/` could possibly invoke a command, when in reality it is being sent verbatim.

### Note

I spoke to `Nei` in `#irssi` on `libera.chat` who advised me to create an issue for this matter.