Return the stdlib error interface

[astrolox]

Thanks for an excellent library!

I'm opening this pull request to propose a several very small changes which are intended to make the Append function "behave as you would expect" (as is suggested in the README).

Changes:

• Return the standard library error interface from Append (this is a change to the function signature)

  Note the following advice from the golang FAQ

  It's a good idea for functions that return errors always to use the error type in their signature (as we did above) rather than a concrete type such as MyError, to help guarantee the error is created correctly. As an example, os.Open returns an error even though, if not nil, it's always of concrete type os.PathError.

• Return the standard library error interface from (g *Group) Wait() (this is a change to the function signature)

• Return nil from Append when there is no error

• Return the original error from Append when there is only one error (i.e. appending to nil)

• Introduce a new Cast method to allow older code which requires a *multierror.Error to be easily updated

• Change the Group.err field from *Error to error

[astrolox]

Fixes #42

[mitchellh]

Thank you for the PR. Up front, this PR unfortunately can't be merged without an associated major version bump (v2). Although the it is a "small change" it is still a breaking change and since we use Go modules and use this library heavily expecting the *Error type to be returned, this would require a major version bump.

Your reasoning on idiomatic Go is completely valid, however we did make this return value the way we did purposefully and thoughtfully based on our call style. We wanted to avoid (or exactly control) the nil is not nil situation and we commonly initialize an empty *Error to setup formatting or some other thing and call append on it throughout (potentially with nil errors).

The function you're looking for is probably ErrorOrNil which is what we always call on return (return merr.ErrorOrNil()) which returns the stdlib error type as you expect. This is safe to call on a nil *Error.

I think if we were building this library from scratch, we may consider changing the semantics as you've proposed. I don't disagree with them. However, in the context of a library that has existed for many years and is already deeply in use in dozens of our own products (and thousands of other downstreams according to pkg.go.dev), I don't think the change justifies the breaking change and major version bump.

[astrolox]

No problem. Thank you for the quick reply. I'll maintain a fork for my own use.