Don't obfuscate fine performance metrics failures

[crusaderky]

It was reported offline that the line

with context_meter.meter("thread-noncpu", func=time) as m:

in worker.py::apply_function_simple failed.
While this should never be possible, it happened for some reason. The way our try...except blocks are written caused the actual stack trace to be swallowed; the user got instead a confusing

  File "distributed/worker.py", line 3015, in apply_function_simple
    msg["start"] = m.start + time_delay
      ^^^^^^^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'm' where it is not associated with a value

This PR causes the actual stack trace detailing the failure of context_meter.meter to show up in the user's logs.

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    27 files  ± 0      27 suites  ±0   10h 3m 20s ⏱️ + 9m 35s
 4 050 tests ± 0   3 934 ✅  -  4    110 💤 ±0   5 ❌ +3  1 🔥 +1 
50 855 runs  +19  48 506 ✅ +21  2 334 💤  - 6  14 ❌ +3  1 🔥 +1 

For more details on these failures and errors, see this check.

Results for commit ad3e016. ± Comparison against base commit 0438768.

[fjetter]

While this should never be possible

IIUC this is interacting with the event loop behind the scenes so everything is possible.