cmd/age-keygen: actually exit on errorf (#383)

Co-authored-by: Arnau Diaz <arnau.diaz@adevinta.com>

cmd/age-keygen/keygen.go

@@ -154,6 +154,7 @@ func convert(in io.Reader, out io.Writer) {
 
 func errorf(format string, v ...interface{}) {
 	log.Printf("age-keygen: error: "+format, v...)
+	log.Fatalf("age-keygen: report unexpected or unhelpful errors at https://filippo.io/age/report")
 }
 
 func warning(msg string) {