Function unmarshal swallows specific error message

[ewohltman]

While recently troubleshooting an issue, I found that it was very useful to include the specific error message in the error returned by the function unmarshal(data []byte, v interface{}) error in restapi.go.

Currently, we swallow the specific error message:

func unmarshal(data []byte, v interface{}) error {
	err := json.Unmarshal(data, v)
	if err != nil {
                // err is effectively lost here
		return ErrJSONUnmarshal
	}

	return nil
}

Instead, we can wrap ErrJSONUnmarshal and include the specific error message:

func unmarshal(data []byte, v interface{}) error {
	err := json.Unmarshal(data, v)
	if err != nil {
		return fmt.Errorf("%w: %s", ErrJSONUnmarshal, err)
	}

	return nil
}

This error wrapping functionality was introduced to the standard library in Go 1.13, so I believe we should update the go.mod file as well. Background: https://blog.golang.org/go1.13-errors

module github.com/bwmarrin/discordgo

go 1.13

require (
	github.com/gorilla/websocket v1.4.2
	golang.org/x/crypto v0.0.0-20210421170649-83a5a9bb288b
)

Fortunately, the CI process only tests 1.13 and later so in a sense we already no longer support Go versions less than 1.13. While we're working on this, and since Go 1.16 has been out for awhile, might as well update the CI to test with 1.16 too:

language: go
go:
    - 1.13.x
    - 1.14.x
    - 1.15.x
    - 1.16.x
env:
    - GO111MODULE=on
install:
    - go get github.com/bwmarrin/discordgo
    - go get -v .
    - go get -v golang.org/x/lint/golint
script:
    - diff <(gofmt -d .) <(echo -n)
    - go vet -x ./...
    - golint -set_exit_status ./...
    - go test -v -race ./...