Cutting the server connection causes unneccessary warnings

From !258 (comment 2119608), cc @pnowack.

This is a cosmetic issue: during ei_disconnect() libei tries to send the various messages (remove device, seats, etc.) as it unwinds but they all fail with the same broken pipe error that caused us to call ei_disconnect() to begin with. The result is a multitude of warnings, one per object basically:

09:55:47 | WARN | failed to send message: Broken pipe

Can be reproduces by running the ei-demo-server and killing it.

Backtrace from one such error:

 09:48:35 | WARN | failed to send message: Broken pipe

Breakpoint 1, ei_send_message (ei=0x4082d0, object=0x409a10, opcode=0, signature=0x7ffff7fb2095 "", nargs=0) at ../src/libei.c:847
847			log_warn(ei, "failed to send message: %s", strerror(-rc));
(gdb) bt
#0  ei_send_message (ei=0x4082d0, object=0x409a10, opcode=0, signature=0x7ffff7fb2095 "", nargs=0) at ../src/libei.c:847
#1  0x00007ffff7f9f438 in ei_device_request_release (ei_device=0x4099f0) at src/ei-proto.c:678
#2  0x00007ffff7fa89ef in ei_device_send_release (device=0x4099f0) at ../src/libei-device.c:1026
#3  0x00007ffff7fa8a9a in ei_device_close (device=0x4099f0) at ../src/libei-device.c:1050
#4  0x00007ffff7faeb1d in ei_seat_remove (seat=0x4095d0) at ../src/libei-seat.c:209
#5  0x00007ffff7fa3f96 in ei_disconnect (ei=0x4082d0) at ../src/libei.c:575
#6  0x00007ffff7fa46b7 in connection_dispatch (source=0x408940, userdata=0x4082d0) at ../src/libei.c:745
#7  0x00007ffff7fb14ec in sink_dispatch (sink=0x408480) at ../src/util-sources.c:178
#8  0x00007ffff7fa3334 in ei_dispatch (ei=0x4082d0) at ../src/libei.c:207
#9  0x000000000040327f in main (argc=2, argv=0x7fffffffdaa8) at ../tools/ei-demo-client.c:302