Add better message for "network error" while uploading changes

How to replicate:

- Start the app connected to the internet
- Choose an existing node
- Turn off any network access (enter flight mode)
- Delete the selected node and confirm
- Go to "Your Contributions" screen

The message related to this error is:

`Unexpected Error: UploadNetworkError"

It seems that there is already a better error message for that, but somehow it is not displaying in this case:

https://github.com/developmentseed/observe/blob/0122d549376cc6e517205a168e96a0b07c378f2e/app/utils/errors.js#L27