improve error quality + test for PR #339

usbSerialForAndroid/src/androidTest/java/com/hoho/android/usbserial/DeviceTest.java

@@ -1846,7 +1846,7 @@ public class DeviceTest {
         try {
             usb.open(EnumSet.of(UsbWrapper.OpenCloseFlags.NO_IOMANAGER_THREAD, UsbWrapper.OpenCloseFlags.NO_DEVICE_CONNECTION));
             fail("open error expected");
-        } catch (IOException ignored) {
+        } catch (Exception ignored) {
         }
 
         usb.open(EnumSet.of(UsbWrapper.OpenCloseFlags.NO_IOMANAGER_THREAD));

usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CommonUsbSerialPort.java

@@ -93,6 +93,9 @@ public abstract class CommonUsbSerialPort implements UsbSerialPort {
         if (mConnection != null) {
             throw new IOException("Already open");
         }
+        if(connection == null) {
+            throw new IllegalArgumentException("Connection is null");
+        }
         mConnection = connection;
         try {
             openInt(connection);