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.
```

caddyconfig/caddyfile/parse.go

@@ -211,6 +211,12 @@ func (p *parser) addresses() error {
 			if expectingAnother {
 				return p.Errf("Expected another address but had '%s' - check for extra comma", tkn)
 			}
+			// Mark this server block as being defined with braces.
+			// This is used to provide a better error message when
+			// the user may have tried to define two server blocks
+			// without having used braces, which are required in
+			// that case.
+			p.block.HasBraces = true
 			break
 		}
 
@@ -571,8 +577,9 @@ func (p *parser) snippetTokens() ([]Token, error) {
 // head of the server block with tokens, which are
 // grouped by segments.
 type ServerBlock struct {
-	Keys     []string
-	Segments []Segment
+	HasBraces bool
+	Keys      []string
+	Segments  []Segment
 }
 
 // DispenseDirective returns a dispenser that contains

caddyconfig/httpcaddyfile/httptype.go

@@ -170,7 +170,11 @@ func (st ServerType) Setup(inputServerBlocks []caddyfile.ServerBlock,
 			dirFunc, ok := registeredDirectives[dir]
 			if !ok {
 				tkn := segment[0]
-				return nil, warnings, fmt.Errorf("%s:%d: unrecognized directive: %s", tkn.File, tkn.Line, dir)
+				message := "%s:%d: unrecognized directive: %s"
+				if !sb.block.HasBraces {
+					message += "\nDid you mean to define a second site? If so, you must use curly braces around each site to separate their configurations."
+				}
+				return nil, warnings, fmt.Errorf(message, tkn.File, tkn.Line, dir)
 			}
 
 			h := Helper{