committing the same version to the same namespace a second time shoudl fail

[stew]

I'm bad at my job and sometimes I say "nelson comit --unit teeth --version 0.0.1 -namespace prod" and then I make version 0.0.2 of my container and I want to put it into prod, so I run the same commit command for 0.0.1 because I suck at my job and now I have two 0.0.1 things competing for the same resources and its all terrible. When I try to commit 0.0.1 to prod and 0.0.1 is already in prod, that should fail

[timperrett]

@stew any chance you can PR this to provide an error message instead?