verify s3 url before running s3_parallel_read

[frschwab]

https://my-bucket.s3-eu-central-1.amazonaws.com
caused an issue with nil values in Lua.
If we verfied the url before trying to run the script, we could have had a better error message, because the real url would be:
https://my-bucket.s3.eu-central-1.amazonaws.com
The 1st url has a - after s3, which is not valid.