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
That's a good point. I spoke to Jon about this once and our general thought was to write new code matching the code style but not to try and tidy up old code as we go. Updating existing code as we went was going to leave a lot of code as half-and-half. We thought we might go through one day and do a 'fix all the code style in one big go' session.