Ref T13528. If we start a subprocess that immediately exits and then write to it, we can get a broken pipe error.
Recent versions of PHP appear to raise this as an actual warning, and recent changes upgrade the warning to a runtime exception.
I can't find any way to tell if the RuntimeException is a broken pipe or something else, except by examining the text of the error string.
At least for now, treat this like a "blocked pipe" condition. Since the subprocess has exited and the bytes didn't write, this should generally be reasonable.