Relax identity server discovery checks to FAIL_PROMPT

[jryans]

As discussed in MSC2284, this relaxes the identity server discovery to a
FAIL_PROMPT state so that clients can choose to warn and continue.

Part of element-hq/element-web#11102
Implements matrix-org/matrix-spec-proposals#2284

[jryans]

Still need to fix the failing tests... As part of that, I am trying find out why the BK log doesn't show the test summary.

[jryans]

Okay, tests should be passing now. I'll continue to explore the test log problem separately.

[turt2live]

We need to get the MSC across the line before we can merge this. Our new spec process doesn't allow us to move quickly for .well-known

[jryans]

Fair enough, I'll mark the issue blocked then.

[jryans]

(It would still be good to review the code, even if we can't merge it yet.)

[turt2live]

code largely seems fine (and yes, I do agree with the changes but the spec process is dumb for cases like this)

[jryans]

@turt2live What phase of the spec process do we need to wait for in this case?

[turt2live]

Release with a formal version number.

[turt2live]

unless we fix the process, of course.

[jryans]

From element-hq/element-web#11102, it was decided it was safe to merge this work once we cleared FCP.

[jryans]

This PR was accidentally forgotten... It doesn't appear to be required for React SDK / Riot to process a dead IS as optional, but that's more of a curiosity of how it interprets discovery results. This work is the key of implementing the MSC text as written and work be important for any other uses of JS SDK.

[jryans]

It turns out this side was more important than I remembered... 😞 In particular, it's needed at least for the startup autodiscovery, so that we don't show a hard error as in element-hq/element-web#11508.