This blog post by Dimitri Merejkowsky clued me into the fact that I had
incorrectly ignored a compiler warning about an unused assignment:
https://dmerej.info/blog/post/awesome-rust-warnings/#the-useless-mutation

When I wrote that line, I didn't realise you could declare an
uninitialised variable, and definitely didn't get what the compiler
meant when it told me:

    warning: value assigned to `email_eid` is never read
       --> src/main.rs:102:9
        |
    102 |     let mut email_eid = String::new();
        |         ^^^^^^^^^^^^^
        |
        = note: `#[warn(unused_assignments)]` on by default
        = help: maybe it is overwritten before being read?

so I ignored the warning.

If there's only one error in the chain, it prints:

    error: no matches for event ID: no matches for event ID

Now, it will print:

    error: no matches for event ID

and:

    error: unable to open OAuth secret file: Not a directory (os error 20)

Previously there were two OAuth authorisation prompts, one for each API
call, because each call has different default scopes.

Since we always need Event read-write access, set this as our scope for
both API calls so that we request only what the application as a whole
needs, and so we don't ask for authorisation twice (which requires
visiting a URL with one's Google account).

Because I was using `xdg_dirs.find_data_file()` to get the path to
"token.json", an error was triggered when the file didn't exist yet. It
doesn't need to exist because the program is supposed to create it, so
we can't use `find_data_file()`.

Remove the `local_data_file()` function, and instead call the XDG
functions directly where they're needed since in one case we need to
find an existing file (and error with a specific message if it doesn't
exist), and in another, we need a path to a file to create or read.

Give more context about the error produced. Trying to figure out how to
only display this if there is a root cause, but I'll work that out
later.