config: Return error from optFns in LoadDefaultConfig

[pingleig]

Built in LoadOptions (e.g. WithRegion) all returns nil. However LoadOptions is public so user can define options that returns error. I find existing code does not ignore error from resolvers or loaders and feel LoadDefaultConfig should do the same.

aws-sdk-go-v2/config/config.go

Lines 136 to 141 in e10c0d2

	for _, fn := range resolvers {
	if err := fn(ctx, &cfg, cs); err != nil {
	// TODO provide better error?
	return aws.Config{}, err
	}
	}

[jasdel]

Thanks for fixing this bug @pingleig. I've added changelog entry for the fix. We'll get this merged in and included in the SDKs next release.