Stack traces in union rules or anything called from a union rule are being swallowed

[ndellosa95]

Describe the bug
When doing plugin development locally, I noticed I stopped getting proper stack traces sometimes and just got

Traceback (most recent call last):
  File "/Users/nick.dellosa/Library/Caches/nce/695a0515ecce6747a3e42742caf246ea42091f11bfbdd8d95ea0903e90eb510a/bindings/venvs/2.19.0rc5/lib/python3.9/site-packages/pants/core/goals/package.py", line 165, in package_asset
    packages = await MultiGet(
  File "/Users/nick.dellosa/Library/Caches/nce/695a0515ecce6747a3e42742caf246ea42091f11bfbdd8d95ea0903e90eb510a/bindings/venvs/2.19.0rc5/lib/python3.9/site-packages/pants/engine/internals/selectors.py", line 374, in MultiGet
    return await _MultiGet(tuple(__arg0))
  File "/Users/nick.dellosa/Library/Caches/nce/695a0515ecce6747a3e42742caf246ea42091f11bfbdd8d95ea0903e90eb510a/bindings/venvs/2.19.0rc5/lib/python3.9/site-packages/pants/engine/internals/selectors.py", line 172, in __await__
    result = yield self.gets
<actual error here>

I was able to do some experimentation and narrow this done to only occurring in union rules or rules called from union rules. And it seems to effect both plugins and backends built into Pants.

You can reproduce this issue by just throwing any Python error from any union rule or rule called from a union rule. Then try it with a goal rule or rule called from a goal rule and you'll see a proper stack trace.

Pants version
2.19.0rc5 and 2.19.0

OS
Intel Mac

Additional info
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