Don't log EPIPE (broken pipe) errors as warnings

Broken pipe errors are usually caused by connection reset, and its better to not log `EPIPE` errors to minimize log spam.

We are running a file serve using tornado, there is tens of thousands of broken pipe errors (in `_handle_write` function) every day.

This is much like the behavior to not log `ECONNRESET` errors in `BaseIOStream._read_to_buffer` function.