Unhelpful error if `curl` is not on the path

**Describe the bug**
If `curl` is not on the path and you attempt to access a `https` repository, then you get an uninformative error `user error (https not supported)`.

Might be worth auditing other process calls, I wouldn't be surprised if "program not found" and "program failed" end up getting the same treatment.

**To Reproduce**
Steps to reproduce the behavior:

Check out https://github.com/input-output-hk/plutus and try any cabal operation without `curl` on your path.

**Expected behavior**
An error like `The program 'curl' was not found on the path and is needed for downloading repositories`