Previously _send_text() caught all exceptions (except Exception) when
sending HTML-formatted messages, falling back to plain text even for
network errors like TimedOut and NetworkError. This caused connection
demand to double during pool exhaustion scenarios (3 retries × 2
fallback attempts = 6 calls per message instead of 3).
Now only catches BadRequest (HTML parse errors), letting network errors
propagate immediately to the retry layer where they belong.
Fixes: HKUDS/nanobot#3050