File upload should raise an exception if the file does not exist on client side

[pablo-de-andres]

In GitLab by @abahde on Jun 10, 2020, 13:53

When I do CUBA.FILE(path=<a path to a file that does not exist> and commit this CUDS object to the dataspace everything seems to be ok. However, in the background, the file is not copied (because it doesnt exist). That will confuse the user, especially when he tries to download the file from the server. We should raise an exception when the file does not exist.