stmhal/accel: Raise an exception if the accel couldn't be initialised.

On PYBLITEv1.0 there is no accelerometer and in this case the Accel()
constructor should not silently succeed.

stmhal/accel.c

@@ -89,15 +89,17 @@ STATIC void accel_start(void) {
 
     HAL_StatusTypeDef status;
 
-    //printf("IsDeviceReady\n");
     for (int i = 0; i < 10; i++) {
         status = HAL_I2C_IsDeviceReady(&I2CHandle1, MMA_ADDR, 10, 200);
-        //printf("  got %d\n", status);
         if (status == HAL_OK) {
             break;
         }
     }
 
+    if (status != HAL_OK) {
+        nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_OSError, "accelerometer not found"));
+    }
+
     // set MMA to active mode
     uint8_t data[1] = {1}; // active mode
     status = HAL_I2C_Mem_Write(&I2CHandle1, MMA_ADDR, MMA_REG_MODE, I2C_MEMADD_SIZE_8BIT, data, 1, 200);