Lack of available read handlers for a read should respond with BUSY, not RESOURCE_EXHAUSTED

[bzbarsky-apple]

Problem

If I do a read and get back RESOURCE_EXHAUSTED, there is no way to tell whether that's because there was another read in progress (in which case I should just retry) or whether it's because my read attempt used too many paths, filters, etc (in which case I need to change what sort of request I am sending).

Proposed Solution

Use BUSY to indicate "no available read clients right now", not RESOURCE_EXHAUSTED.

[bzbarsky-apple]

Needs a spec-side adjustment too: https://github.com/CHIP-Specifications/connectedhomeip-spec/issues/5198

[bzbarsky-apple]

https://github.com/CHIP-Specifications/connectedhomeip-spec/issues/5198 is now fixed. So this just needs code changes.