httpcaddyfile: Improve error on matcher declared outside site block

[francislavoie]

We don't support defining matchers outside of site blocks. Some users try it, but get confusing error messaging because it still gets read as a site address. Instead, we can improve the error message in those cases because a site address starting with @ isn't valid anyways.

Consider this Caddyfile:

@foo {
	path /bar/*
}

localhost {
	respond "hello"
}

Before:

$ ./caddy adapt --config Caddyfile
adapt: Caddyfile:2: unrecognized directive: path

After:

$ ./caddy adapt --config Caddyfile
adapt: cannot define a matcher outside of a site block: '@foo'

[mholt]

I like the idea of a specialized error message, but this is the wrong implementation: it forbids any Caddyfile format from having keys that start with @ (imagine, for example, an email server Caddyfile, or a Twitter something-or-other Caddyfile). This change should be made within the httpcaddyfile package.

[francislavoie]

Good point. Updated.

Doing it in httpcaddyfile means we lose information about the current line and filename we're on though, but no big deal.

[mholt]

Cool, thanks!