Fix ClientSession swallowing exceptions in default message_handler

[mayuriphad]

Fixes #1401 by raising \Exception\ messages passed into the default \message_handler\ instead of swallowing them. This ensures stream errors propagate out of the \ClientSession\ by default.

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[github-actions]

Thanks for the contribution. This repository only keeps pull requests open when they're linked to an issue that a maintainer has assigned to the author — CONTRIBUTING.md explains why and how we work. This PR has been closed for now because you aren't currently assigned to #1401.

If a maintainer would like this change as a PR from you, they'll assign you to #1401 and this PR will reopen automatically — there's nothing more you need to do. (If you opened the issue, this PR already shows up on its timeline.)

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