Raise an error if npm install fails during "install runtime" mix task

[taylordowns2000]

The System.cmd() in lib/mix/engine.install.runtime.ex should be wrapped in a case statement that checks for the exit of the npm install command then raises an error if it fails. Oterhwise, folks could run mix openfn.install.runtime and get a false success due to npm errors.

    System.cmd(
      "/usr/bin/env",
      [
        "sh",
        "-c",
        "npm install --prefix $NODE_PATH --global #{package_list}"
      ],
      env: [{"NODE_PATH", @default_path}],
      stderr_to_stdout: true,
      into: IO.stream(:stdio, :line)
    )