Yet another errors package for Go.

Import as zgo.at/errors; API docs: https://godocs.io/zgo.at/errors

This is based on the new errors API introduced in Go 1.13 with the additions:

1. Add Wrap(err, "some context") and Wrapf(err, "some context", ...), which return nil if the passed error is nil.

   I tried using the fmt.Errorf("...: %w", err) pattern, but I found I missed being able to do:

   func f() error {
       return errors.Wrap(f2(), "context")
   }
   

   Which I find much more convenient than:

   func f() error {
       err := f2()
       if err != nil {
           return fmt.Errorf("context: %w", err)
       }
       return nil
   }
   

2. A stack trace is added with erorrs.New(), errors.Errorf(), and erorrs.Wrap() I know it's not needed with appropriate context but sometimes I accidentally add the same context more than once, or just want to quickly see where exactly the error is coming from. Especially on dev it's much more convenient.

   You can use errors.Package to only add stack traces for a specific package. For example errors.Package = "zgo.at/goatcounter" will only add trace lines in zgo.at/goatcounter/...

   You can control the maximum stack size with errors.StackSize; set to 0 to disable adding stack traces altogether (i.e. on production).

3. Group type for collecting grouped errors:

    errs := NewGroup(20)  // 20 is the maximum amount of errors
    for {
        err := doStuff()
        if err.Append(err) { // Won't append on nil, returns false if it's nil.
            continue
        }
    }
   
    fmt.Println(errs)        // Print all errors (if any).
    return errs.ErrorOrNil() // No errors? Returns nil.