Better handle rest errors on explorer upload API

Current api used for rest calls during automation are originally designed for the webengine UI.
When an error happens on upload, the distribution might be partially imported. The page redirects to a webengine template explaining that there was an error but:

• the request is still a 202 --> curl cannot detect the issue
• the upload is done partially, this should not be the case with proper core session saving and transaction rollback

Similar redirect after save should also be improved (although errors are more likely to happen on upload, the same redirect html page is given to curl, which is not ideal when automating save)