Add opt-in to cache responses with errors

[lwasyl]

Most straightforward way to fix #2358 by implementing #2281 (comment).

I could make STORE_PARTIAL_RESPONSES deprecated right away, if there's another suggestion to enable old behavior. Overall I consider this a quick workaround for people affected by the breaking change introduced in #2281

[tasomaniac]

Looks good. I actually think that this is the right way to do it but not a work-around. The behavior change in 2.1.0 was also discussed with apollo-ios folks. Storing partial when errors are present brings some other edge case issues that are hard to resolve. Enabling that via header is a good solution for people need it.

My question is that how can we make this more obvious in the documentation?

[martinbonnin]

I too like the flexibility of this solution. I also like that the default is to not cache partial responses.

I'm not sure we want to want to advertise the new header too much though. It's an advanced use case and I'd prefer users to not use errors for normal operation. Having to dig a bit for the header would ensure that users are aware of what they are doing ?

[tasomaniac]

If they're aware of the custom header support, then these docs on the objects are all good. I'm just afraid that not a lot of people know that this even exists.

[martinbonnin]

@tasomaniac I can write some doc about this. I think we need to elaborate on the CacheKeyResolver documentation. I was thinking about doing a separate Normalized Cache page and there could be a section about partial responses. Would that work?

@sav007 any objection to merging this?

[tasomaniac]

@martinbonnin up to you. You know it the best. I just wanted to bring it up. If we don't have the right context to document this particular behavior we can just mention the fact that we have options to control caching behavior.