cgo: do not reset tag generator between files

Clean up an error message and error exit too.
Insert blank line after "DO NOT EDIT" comment
to keep it from being a doc comment.

Fixes #1213.
Fixes #1222.

R=r
CC=golang-dev
https://golang.org/cl/3608042

src/cmd/cgo/main.go

@@ -219,6 +219,9 @@ func main() {
 	}
 
 	p.writeDefs()
+	if nerrors > 0 {
+		os.Exit(2)
+	}
 }
 
 // Record what needs to be recorded about f.