Check uploads directory is writable before beginning

HMCI should perform a `is_writable()` check on the uploads directory before even beginning an import and bail out straight away if it's not.

Trace the import entry point and the code that determines the uploads directory. Start by checking how imports currently begin and how directory errors are handled; done means a non-writable uploads directory is detected before import work starts and the import exits immediately.

Written by the indexing model from the issue text.