Feature request: Make the on-error value persistent

[g1za]

Is your feature request related to a problem? Please describe.
In case of a scraping error the 'last' value is retained, but only until a reboot.
At that point the sensor value becomes "unavailable" and any attributes are not available as well.
This situation persists until the scraping is successful.

Describe the solution you'd like
On error, it would be nice to make 'last' value persistent to prevent to lose it in case of a system reboot.
Attributes as well if possible.

Describe alternatives you've considered
A workaround is to manually create a template sensor that receives the value from the scarping sensor and updates it only if the state is not "unavailable".