DropboxManager: Add err message if passing an invalid fd to addFile()

Follow up to I076bfd3180fb9b4baff7e1bae2e611419061b2a7. Adds an
error message if passing -1 to addFile(int fd)

Change-Id: I73a8d88f12b14bc28ea3bc3782a9df7d96d53c92
Test: builds
1 file changed