Client error checking

Should fully address #136 (upload existing deleted file).

Partially addresses #203 (disabling 'OK' buttons on errors). Currently this is in place for actions and events. The other ones should be similar, but I figured I'd give people a chance to object to the method before I continue. I'll update the pull request tomorrow with the other dialogs as I go. 

One of the things I don't like about the solution to #203 is that I'm adding wrappers for the error check functions to the ```WAVEDViewModel```. If I'm missing an obviously better method to do this please let me know (though this is similar to things like ```selectedActionName.error``` being in ```WAVEDViewModel``` which also makes it feel a bit cluttered)