Migrate error values in fritzbox_dsl.py to use API

[ma4nn]

In order to fully migrate fritzbox_dsl.py script from scraping the data.lua page to the TR-064 API, the error values have to be mapped and at the moment it is unclear which of the old ones map to the values returned by the API:

Mode	Title in Munin Graph	Title in data.lua	TR-064 API
errors	Transmission Errors	Fehlern (ES) + Fehlern (SES)
crc	Checksum Errors	Nicht behebbare Fehler (CRC)
ecc	Error Correction	korrigierte DTU

Perhaps someone can shed some light on this as I am not too much into the details of these different kinds of errors.