Fix invalid type assertion in LoadBytes (#733)

Fixes #732

marshal_test.go

@@ -4119,3 +4119,13 @@ ErrorField = "foo"
 		t.Fatalf("error was expected")
 	}
 }
+
+func TestGithubIssue732(t *testing.T) {
+	var v interface{}
+	data := []byte("a=\nb=0")
+
+	err := Unmarshal(data, &v)
+	if err == nil {
+		t.Fatalf("error was expected")
+	}
+}

toml.go

@@ -471,7 +471,7 @@ func LoadBytes(b []byte) (tree *Tree, err error) {
 			if _, ok := r.(runtime.Error); ok {
 				panic(r)
 			}
-			err = errors.New(r.(string))
+			err = fmt.Errorf("%s", r)
 		}
 	}()