Return specific error types

[cjmarkham]

Given the following section of code in the GetAst method:

if err != nil {
	return nil, fmt.Errorf("could not parse filter:%w", err)
} else if err := astNode.checkValid(); err != nil {
	return nil, fmt.Errorf("error validating filter:%w", err)
} else {
	return astNode, nil
}

It returns an error using fmt.Errorf. This makes it hard to determine the type of error (either parsing or validation error).

If it were replaced with specific error types, we could handle these errors in the calling functions.

For example:

type ParsingErr struct {
	err error
}
func (pe ParsingErr) Error() string {
	return pe.err.Error()
}
type ValidationErr struct {
	err error
}
func (ve ValidationErr) Error() string {
	return ve.err.Error()
}

func CouldNotParse(err error) ParsingErr {
	return ParsingErr{
		err: fmt.Errorf("could not parse filter: %w", err),
	}
}

func CouldNotValidate(err error) ValidationErr {
	return ValidationErr{
		err: fmt.Errorf("error validating filter:%w", err),
	}
}

And then the caller could use something like

ast, err = epsearchastv3.GetAst(ast)
switch {
case errors.As(err, &epsearchastv3.ParsingErr{}):
	// Handle validation err
case err != nil:
	// handle other err
}