Emit error tokens when there's a group mismatch.

Also don't panic/recover, as we no longer use panic to report "real"
errors.

Fixes #295.

formatters/html/html_test.go

@@ -2,7 +2,6 @@ package html
 
 import (
 	"bytes"
-	"errors"
 	"io/ioutil"
 	"strings"
 	"testing"
@@ -50,14 +49,6 @@ func TestSplitTokensIntoLines(t *testing.T) {
 	assert.Equal(t, expected, actual)
 }
 
-func TestIteratorPanicRecovery(t *testing.T) {
-	it := func() chroma.Token {
-		panic(errors.New("bad"))
-	}
-	err := New().Format(ioutil.Discard, styles.Fallback, it)
-	assert.Error(t, err)
-}
-
 func TestFormatterStyleToCSS(t *testing.T) {
 	builder := styles.Get("github").Builder()
 	builder.Add(chroma.LineHighlight, "bg:#ffffcc")