Creating client without IPFS_PATH leads to nil

[hinshun]

If the default ~/.ipfs, IPFS_PATH env var, or ipfspath function arg passed to NewPathApi is a non-existent directory, it will return nil error and nil client.

A common pattern in golang is this:

cln, err := httpapi.NewLocalApi()
if err != nil {
  return err
}

// cln.Unixfs().Add ...

But that becomes a nil pointer reference. Curious why os.IsNotExist errors are hidden here:
https://github.com/ipfs/go-ipfs-http-client/blob/master/api.go#L56-L59

[Stebalien]

This probably comes from the fact that this function used to return no error. I agree we should return some kind of ErrNotRunning.