caddyfile: Better error message for missing site block braces (#4301)

Some new users mistakenly try to define two sites without braces around each. Doing this can yield a confusing error message saying that their site address is an "unknown directive".

We can do better by keeping track of whether the current site block was parsed with or without a brace, then changing the error message later based on that.

For example, now this invalid config:

```
foo.example.com
respond "foo"

bar.example.com
respond "bar"
```

Will yield this error message:

```
$ caddy adapt
2021/08/22 19:21:31.028 INFO    using adjacent Caddyfile
adapt: Caddyfile:4: unrecognized directive: bar.example.com
Did you mean to define a second site? If so, you must use curly braces around each site to separate their configurations.
```

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