add check for cargo (#57)

computer-use-demo/setup.sh

@@ -1,4 +1,10 @@
 #!/bin/bash
+if ! command -v cargo &> /dev/null; then
+    echo "Cargo (the package manager for Rust) is not present.  This is required for one of this module's dependencies."
+    echo "See https://www.rust-lang.org/tools/install for installation instructions."
+    exit 1
+fi
+
 python3 -m venv .venv
 source .venv/bin/activate
 pip install --upgrade pip