Errors go to stdout and don't produce nonzero exit status

This construct occurs a bunch of times:

```go
if err != nil {
fmt.Println(err)
return nil
}
```

But:

1. This prints errors to stdout instead of stderr (similar to #146).
2. It swallows the error rather than making the program exit with a nonzero status.

Both things make oauth2l unsuitable for use in scripts in particular.

A naive fix would be to replace all these constructs with `panic(err.Error())` but it would make more sense to rethrow (well, re-return) errors and add a global error handler that calls `os.Exit(code)`, optionally with a specific code per error type.

I'd be willing to add the `panic` calls in a PR but I don't think I understand the codebase enough to start a more aggressive refactor.