Five ``except Exception as exc:`` blocks in the Matrix adapter logged
only ``str(exc)`` without ``exc_info=True``:
- _reverify_keys_after_upload → post-upload key verification failure
- _upload_keys_if_needed → initial device-key query failure
- _upload_keys_if_needed → re-upload device keys failure
- _upload_keys_if_needed → initial device key upload failure
- connect → whoami / access-token validation failure
The E2EE key paths here are security-critical: a silent traceback-
less failure during device-key verification or upload makes it
hard for operators to tell whether their Matrix bot is failing
because of a stale token, a federation timeout, or an olm state
mismatch — all three fail with different tracebacks, which
``str(exc)`` alone flattens.
The contributing guide asks for ``exc_info=True`` on error logs.
Append it to each of the five call sites. Pure logging enrichment.
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