refactor: make EmitLoop error message clearer

[tlambert03]

trying to make tracebacks in emit loop errors slightly clearer:

Traceback (most recent call last):
  File "/Users/talley/dev/self/psygnal/src/psygnal/_signal.py", line 990, in _run_emit_loop
    caller.cb(args)
  File "/Users/talley/dev/self/psygnal/src/psygnal/_weak_callback.py", line 324, in cb
    self._f(*self._args, *args, **self._kwargs)
  File "/Users/talley/dev/self/psygnal/x.py", line 22, in f1
    f2()
  File "/Users/talley/dev/self/psygnal/x.py", line 26, in f2
    f3()
  File "/Users/talley/dev/self/psygnal/x.py", line 30, in f3
    raise ValueError("oops")
ValueError: oops

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/talley/dev/self/psygnal/x.py", line 34, in <module>
    thing.sig.emit(1)
  File "/Users/talley/dev/self/psygnal/src/psygnal/_signal.py", line 945, in emit
    self._run_emit_loop(args)
  File "/Users/talley/dev/self/psygnal/src/psygnal/_signal.py", line 992, in _run_emit_loop
    raise EmitLoopError(
psygnal._exceptions.EmitLoopError:
While emitting signal '__main__.Thing.sig', an error occurred in callback '__main__.f1'.
The args passed to the callback were: (1,)
This is not a bug in psygnal.  See 'ValueError' above for details.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b900ff2) 99.94% compared to head (998b6ce) 99.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   99.94%   99.89%   -0.06%     
==========================================
  Files          22       22              
  Lines        1873     1900      +27     
==========================================
+ Hits         1872     1898      +26     
- Misses          1        2       +1     

Files	Coverage Δ
src/psygnal/_exceptions.py	100.00% <100.00%> (ø)
src/psygnal/_queue.py	100.00% <ø> (ø)
src/psygnal/_signal.py	100.00% <ø> (ø)
src/psygnal/_weak_callback.py	98.98% <100.00%> (-0.47%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #232 will degrade performances by 10.6%

_{Comparing tlambert03:slot-tb (998b6ce) with main (b900ff2)}

Summary

⚡ 2 improvements
❌ 1 (👁 1) regressions
✅ 63 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	tlambert03:slot-tb	Change
⚡	test_emit_time[lambda-2]	123.5 µs	110.9 µs	+11.41%
👁	test_connect_time[setattr-check_types]	124.8 µs	139.6 µs	-10.6%
⚡	test_emit_time[partial_method-2]	136.3 µs	122.9 µs	+10.94%