Handle nil pointer if HNS network not created

[MikeZappa87]

I got the error below and wanted to handle it a little better. It took me awhile to figure out why this was happening. I don't believe the error returned from 'GetNetworkByName' is super user friendly. I can return a useful error message if desired.

Stack trace:

    // time="2022-07-14T03:31:57Z" level=fatal msg="run pod sandbox: rpc error: code = Unknown desc = failed to setup network for sandbox \"1a690258599b6a720007e10571d4ff5b1c46b03f11d34386a3120dd2af7412ef\": plugin type=\"sdnbridge\" name=\"winl2bridgetest\" failed (add): netplugin failed: \"panic: runtime error: invalid memory address or nil pointer dereference\\n[signal 0xc0000005 code=0x0 addr=0x20 pc=0x8b8975]\\n\\ngoroutine 1 [running]:\\ngithub.com/Microsoft/windows-container-networking/common/core.(*netPlugin).Add(0xc000094900, 0xc00005c150, 0x0, 0x0)\\n\\t/home/debjit/goroot/src/github.com/microsoft/windows-container-networking/common/core/network.go:150 +0x655\\ngithub.com/containernetworking/cni/pkg/skel.(*dispatcher).checkVersionAndCall(0xc00025fe58, 0xc00005c150, 0x9b2440, 0xc000048510, 0xc00025fe40, 0xc000048510)\\n\\t/home/debjit/goroot/src/github.com/microsoft/windows-container-networking/vendor/github.com/containernetworking/cni/pkg/skel/skel.go:166 +0x2c4\\ngithub.com/containernetworking/cni/pkg/skel.(*dispatcher).pluginMain(0xc00025fe58, 0xc00025fe40, 0x0, 0xc00025fe28, 0x9b2440, 0xc000048510, 0x95331c, 0x11, 0xc0001511f0)\\n\\t/home/debjit/goroot/src/github.com/microsoft/windows-container-networking/vendor/github.com/containernetworking/cni/pkg/skel/skel.go:218 +0x428\\ngithub.com/containernetworking/cni/pkg/skel.PluginMainWithError(...)\\n\\t/home/debjit/goroot/src/github.com/microsoft/windows-container-networking/vendor/github.com/containernetworking/cni/pkg/skel/skel.go:275\\ngithub.com/Microsoft/windows-container-networking/cni.(*Plugin).Execute(0xc0000c8050, 0x9b2400, 0xc000094900, 0x0, 0xc0000c8048)\\n\\t/home/debjit/goroot/src/github.com/microsoft/windows-container-networking/cni/plugin.go:49 +0x1f8\\ngithub.com/Microsoft/windows-container-networking/common/core.Core()\\n\\t/home/debjit/goroot/src/github.com/microsoft/windows-container-networking/common/core/core.go:50 +0x2e5\\nmain.main()\\n\\t/home/debjit/goroot/src/github.com/microsoft/windows-container-networking/plugins/sdnbridge/sdnbridge_windows.go:16 +0x27\\n\""