scip v0.2.0: CLI improvements with better errors and help docs

Command to download the CLI for your current platform:

env \
  TAG="v0.2.0" \
  OS="$(uname -s | tr '[:upper:]' '[:lower:]')" \
  ARCH="$(uname -m | sed -e 's/x86_64/amd64/')" \
  bash -c 'curl -L "https://github.com/sourcegraph/scip/releases/download/$TAG/scip-$OS-$ARCH.tar.gz"' \
| tar xzf - scip