[Feature]: ics fetch on error keep last valid state

[BelphegorPrime]

I propose a feature for:

The integration in general

Describe your wanted feature

Hi I have the following problem, my source proveider did something with their servers.

for the following domain za-sws.de, that worked until yesterday now only receivs an 503 error.

So when you read it it might be fixed, but my suggestion would be when it is tried to fetch the data and we receive an not Okish response we do nothing.

Because right now every entry was removed from my calendar. :( (even that I had all values for this year, that still would be valid)

Greetings

[Big-Gremlin]

I have the same problem, also with za-sws.de. I would be happy about a corresponding enhancement to keep the latest valid status.