If EINTR caused by SIGINT is encountered while writing to the
`fd_output_stream_t` output fd, mark the output stream as errored and return
false to the caller but do not visibly complain.
Addressing the outstanding TODO notwithstanding, this is needed to avoid
littering the tty with spurious errors when the user hits Ctrl-C to abort a
long-running builtin's output (w/ the primary example being `history`).

When there are multiple screens worth of output and `history` is writing to the
pager, pressing Ctrl-C at the end of a screen doesn't exit the pager (`q` is
needed for that) but previously caused fish to emit an error ("write:
Interrupted system call) until we starting silently handling SIGINT in
`fd_output_stream_t::append()`.
This patch makes `history` detect when the `append()` call returns with an error
and causes it to end early rather than repeatedly trying (and failing) to write
to the output stream.