Issue #119

Fixes 2 problems associated with #119

1) Trying to create a new tag/sensor without a serial number no longer gives you a Grails error page
2) You can no longer create/update a sensor to a type that conflicts with the other sensors on the tag
Is this an opportunity to change the code style to 
 if(something) {
    .....
insead of 
if (something)
{
    .....