Partially revert "unconditionally log request error"

There is not need to excessively produce high-severity logs, while
there are ways to reach that point from entirely benign network errors.

This partially reverts commit 0b10cba.

gunicorn/workers/base.py

@@ -264,7 +264,7 @@ def handle_error(self, req, client, addr, exc):
  if hasattr(req, "uri"):
  self.log.exception("Error handling request %s", req.uri)
  else:
- self.log.exception("Error handling request (no URI read)")
+ self.log.debug("Error handling request (no URI read)")
  status_int = 500
  reason = "Internal Server Error"
  mesg = ""