[hal] Add warning about onboard I2C (#3871)

Adds HAL layer warning for #3842. This is needed in the case when a
vendor uses the HAL directly rather than using the WPILib I2C class.

This should not result in a duplicate warning for WPILib I2C users due
to the duplicate message checking performed in HAL_SendError().

We don't want to remove the WPILib I2C warning because it gives stack
trace information while the HAL layer one can't.

hal/src/main/native/athena/I2C.cpp

@@ -54,6 +54,10 @@ void HAL_InitializeI2C(HAL_I2CPort port, int32_t* status) {
   }
 
   if (port == HAL_I2C_kOnboard) {
+    HAL_SendError(0, 0, 0,
+                  "Onboard I2C port is subject to system lockups. See Known "
+                  "Issues page for details",
+                  "", "", true);
     std::scoped_lock lock(digitalI2COnBoardMutex);
     i2COnboardObjCount++;
     if (i2COnboardObjCount > 1) {