Filter out the same socket-closing errors on flush as on write
Filter out the same socket-closing errors on flush as on write

Before this change, if a broken-pipe or similar error happened in the
call to `flush()` instead of `raw_print()`, it would not be suppressed
but instead surfaced to the caller.

frewsxcv/tiny-http,frewsxcv/tiny-http,tiny-http/tiny-http