text/template: handle panic values that are not errors.

The recover code assumes that the panic() argument was
an error, but it is usually a simple string.
Fixes #2663.

R=golang-dev, r, r, gri
CC=golang-dev, remy
https://golang.org/cl/5527046

src/pkg/text/template/exec.go

@@ -78,10 +78,14 @@ func (s *state) error(err error) {
 func errRecover(errp *error) {
 	e := recover()
 	if e != nil {
-		if _, ok := e.(runtime.Error); ok {
+		switch err := e.(type) {
+		case runtime.Error:
+			panic(e)
+		case error:
+			*errp = err
+		default:
 			panic(e)
 		}
-		*errp = e.(error)
 	}
 }