Fix #3983: Uploads: NoMethodError - undefined method `>=' for nil:NilClass

* Rename Upload#download_for_upload to #get_file_for_upload.

* Fix #get_file_for_upload to raise error if no file or source url was given.

* Fix javascript upload validation to disallow submitting form if file is
  not present and the source is not an url.

app/models/upload.rb

@@ -115,11 +115,11 @@ class Upload < ApplicationRecord
     end
 
     def is_duplicate?
-      status =~ /duplicate: \d+/
+      status.match?(/duplicate: \d+/)
     end
 
     def is_errored?
-      status =~ /error:/
+      status.match?(/error:/)
     end
 
     def sanitized_status