Return an error if file already exists on upload

[c10l]

This is a simple patch that will return an error when trying to upload a module that already exists on the module_dir.

[i11]

Great stuff! Would you mind including a simple unit test to stress added feature? Thanks!

[i11]

Just pushed initial unit test for directory backend class. It would be great if you could just update it with the upload method test.

https://github.com/unibet/puppet-forge-server/blob/master/spec/unit/backends/directory_spec.rb

[c10l]

Added. Let me know if you want me to add or change anything. Thanks for the gem, it's serving me very well! :)

[i11]

Thanks a lot! Merging.