Check for required attributes #9
Description
Seems that some of the attributes seem to be required. At least: uploadUrl, sessionId. Are any other attributes required, too?
Required attributes should be marked in README.md. Their presence should be checked in code before trying to upload. There should be a clear error message if they are missing.
This would help investigating https://github.com/StarcounterApps/Images/pull/99
cc @miyconst