improve error messages in case of parsing errors

[nitram509]

Motivation

The scanner.Next() just returns simple error message, like e.g. "Bad input ...".
See:

feel/scan.go

Line 188 in b5bde4e

func (scanner *Scanner) Next() error {

Since we have more context in the parser, it would be great and more user friendly to give more meaningful error message,
helping in quicker fixing the issue.
E.g. Parser:

feel/parser.go

Line 91 in b5bde4e

err = p.scanner.Next()

Hints/Tasks

• build a concept how to improve error messages inside the parser
• reworking error handling
• create some tests to show case