io_expander: check I2C (n)ack on write

src/libboard_artiq/src/io_expander.rs

@@ -111,9 +111,15 @@ impl IoExpander {
 
     fn write(&self, i2c: &mut i2c::I2c, addr: u8, value: u8) -> Result<(), &'static str> {
         i2c.start()?;
-        i2c.write(self.address)?;
+        if !i2c.write(self.address)? {
-        i2c.write(addr)?;
+            return Err("io expander failed to ack control byte (read)");
-        i2c.write(value)?;
+        }
+        if !i2c.write(addr)? {
+            return Err("io expander failed to ack register address");
+        }
+        if !i2c.write(value)? {
+            return Err("io expander failed to ack value");
+        }
         i2c.stop()?;
         Ok(())
     }