Inconsistent postponement on exceptions

[maxfischer2781]

Description of the bug
The asynchronous Resource.claim operation does not postpone when claiming fails. This violates usim's guarantee to always postpone on an asynchronous operation.

This issue may apply to other operations as well.

How To Reproduce
Performing an impossible claim does not allow other activities to run.

from usim import instant, run, Resources, ResourcesUnavailable

async def fail_postponed():
    res = Resources(capacity=1)
    try:
        async with res.claim(capacity=2):
            pass
    except ResourcesUnavailable:
        print("Postponed?")

async def postponed():
    print("Before postponement...")
    await instant
    print("After postponement...")

run(fail_postponed(), postponed())

Expected behavior
Any asynchronous operation should postpone at least once before raising a non-failure Exception.