vp-uploadfit: don't overwrite fit unless --force specified

Created by: voisey

Closes https://github.com/NNPDF/nnpdf/issues/92

This is a prototype for the above. I'm sure this can be made cleaner, e.g. perhaps the code that checks whether a fit already exists on the server should be put in its own function, but I would like general feedback on whether the general idea is okay before spending time on improving it