Add error handling function to match error codes in nested API errors

[gdavison]

Description

Some AWS API errors return a fairly generic error code with a nested specific code. Currently we only check the top-level error code, and sometimes check the error message. AWS strongly discourages checking the error message, since it can change.

Using the current functions, this is how a specific error message can be checked for

if tfawserr.ErrCodeEquals(err, storagegateway.ErrCodeInvalidGatewayRequestException) {
	var awsErr awserr.Error
	if errors.As(err, &awsErr) {
		nestedErr := awsErr.OrigErr()
		if nestedErr != nil && tfawserr.ErrCodeEquals(nestedErr, "FileSystemAssociationNotFound") {
			return nil, FsxFileSystemStatusNotFound, nil
		}
	}
}

Potential Library Implementation

func ErrCodeEqualsNested(err error, code string) (awserr.Error, bool)

The function should loop until the error code matches or err.OrigErr() returns nil. The awserr.Error return value will be the matched error or nil. The bool return value will be true if there is a match.

References

hashicorp/terraform-provider-aws#20082 (comment)