Return the original error when binding a json payload

[rv-rsouza]

I need to treat some binding errors, like invalid time format when unmarshaling json from the payload.

It wold be nice to have a property on the echo.HTTPError with the original error reference. Something like InnerError (inner exception, as in .Net) or OriginalError.

This would be useful to know what caused the HTTPError aside from looking at the error message. Eg:

var myType MyType
err := c.Bind(&myType)
if err != nil {
    if parseError, ok := err.OriginalError.(json.UnmarshalTypeError); ok {
        fmt.Println("Error parsing field:", parseError.Field)
    }
    // ...
}
// ...

[vishr]

Fixed in #1164