Gracefully handle case where the given room ID on the CLI is not valid

Nobody has claimed this yet.

Of all the things, I know this is probably... okay, *nonexistent* on the priority queue, but.

When an invalid / non-existent room ID is given to the `-r` option, the tool crashes:

```
# ./synapse_compress_state -p ... -r '!this_is:not.valid'
Fetching state from DB for room '!this_is:not.valid'...
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/database.rs:162:22
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

I learned this when my monthly clean-up script was wrapping room IDs in double quotes, and everything was... unceremoniously failing. It seems like, as just a small user experience enhancement, it might be a good idea to catch this case and gracefully inform that `Room '!this_is:not.valid' doesn't exist.`

Start at src/database.rs:162 and reproduce the failure by invoking the tool with the CLI -r option and a nonexistent room ID. The work is done when that input no longer panics and instead reports that the specified room does not exist.

Written by the indexing model from the issue text.