Now fake S3 storage is used if CSOL_AWS_FAKE_S3_DIR is specified.

I also added some code that checks to see if the required CSOL_AWS_*
env vars are defined; if they're not, it throws a helpful error and
mentions that CSOL_AWS_FAKE_S3_DIR can be used as an alternative.

Unfortunately I haven't been able to test out this integration yet,
because I'm having trouble figuring out how to upload evidence locally on
the website...