Handle failures when running NuGet (exitCode != 0, output to stderr)

Previously the script Run-NuGet.ps1 ignored the process exit code so that errors were not propagated to the caller.

This change throws an error when either the process exits with a non-zero value or if errors are written to stderr, which makes Chocolatey more suitable for being called from other scripts.

For example, I'm using Chocolatey to install and build a Linux virtual machine on Windows with a single PowerShell command (https://github.com/webcoyote/linux-vm). Chocolatey is awesome for projects like this!