text parser: improve handling of decode errors

[joachimmetz]

Per: #3298 some input might be malformed. To better handle such corrupt input (1) define an encoding error handler to escape the incorrectly encoded data and (2) track a parse error

• Change dfVFS text file helper to support an error handler Changed text file helper to use decode error handler dfvfs#508
• Change text parser to define and use an encoding error handler

For now do not allow a decode error during validation.