ebnflint: exit with non-zero status on error

Tweak spec to avoid ebnflint complaints.

R=gri
CC=golang-dev
https://golang.org/cl/3973050

doc/go_spec.html

@@ -4078,11 +4078,11 @@ SelectStmt = "select" "{" { CommClause } "}" .
 CommClause = CommCase ":" { Statement ";" } .
 CommCase = "case" ( SendExpr | RecvExpr) | "default" .
 SendExpr =  Expression "<-" Expression .
+RecvExpr =  [ Expression ( "=" | ":=" ) ] "<-" Expression .
+</pre>
 <!-- TODO(rsc):
 RecvExpr =  [ Expression [ "," Expression ] ( "=" | ":=" ) ] "&lt;-" Expression .
 -->
-RecvExpr =  [ Expression ( "=" | ":=" ) ] "&lt;-" Expression .
-</pre>
 
 <p>
 For all the send and receive expressions in the "select"

src/cmd/ebnflint/ebnflint.go

@@ -88,6 +88,7 @@ func main() {
 	src, err := ioutil.ReadFile(filename)
 	if err != nil {
 		scanner.PrintError(os.Stderr, err)
+		os.Exit(1)
 	}
 
 	if path.Ext(filename) == ".html" {
@@ -97,9 +98,11 @@ func main() {
 	grammar, err := ebnf.Parse(fset, filename, src)
 	if err != nil {
 		scanner.PrintError(os.Stderr, err)
+		os.Exit(1)
 	}
 
 	if err = ebnf.Verify(fset, grammar, *start); err != nil {
 		scanner.PrintError(os.Stderr, err)
+		os.Exit(1)
 	}
 }