Improve error message for custom capture

If custom capture generate an error, then user will receive an error message with full struct name.

```
diff --git a/_examples/ini/main.go b/_examples/ini/main.go
index be6ec9e..e483923 100644
--- a/_examples/ini/main.go
+++ b/_examples/ini/main.go
@@ -1,6 +1,7 @@
package main

import (
+ "errors"
"os"

"github.com/alecthomas/repr"
@@ -51,11 +52,17 @@ type String struct {
func (String) value() {}

type Number struct {
- Number float64 `@Float`
+ Number N `@Float`
}

func (Number) value() {}

+type N struct{}
+
+func (*N) Capture(values []string) error {
+ return errors.New("problem")
+}
+

```

`
panic: Number.Number: problem
`

An error message with type name or token name will be much better:
`: problem`