improved the behavior of the code without a gyro board

It's annoying to have it leave lots of coredumps when there isn't a gyro
board plugged in, so I changed it to just LOG(ERROR) and then exit(1)
diff --git a/frc971/input/gyro_sensor_receiver.cc b/frc971/input/gyro_sensor_receiver.cc
index 3d95758..4dec23e 100644
--- a/frc971/input/gyro_sensor_receiver.cc
+++ b/frc971/input/gyro_sensor_receiver.cc
@@ -238,7 +238,8 @@
     dev_handle_ = ::std::unique_ptr<LibUSBDeviceHandle>(
         libusb_.FindDeviceWithVIDPID(kVid, kPid));
     if (!dev_handle_) {
-      LOG(FATAL, "couldn't find device\n");
+      LOG(ERROR, "couldn't find device. exiting\n");
+      exit(1);
     }
     transfer1_ = ::std::unique_ptr<libusb::Transfer>(
         new libusb::Transfer(kDataLength, StaticTransferCallback, this));