Fixing compiler warning and adding some error handling

Fixing compiler warning and adding some error handling

Author: PabloNunes

02-SettingUp.NETDev/src/BasicChat-01MEAI/Program.cs

@@ -4,7 +4,7 @@
 
 IChatClient client = new ChatCompletionsClient(
         endpoint: new Uri("https://models.inference.ai.azure.com"),
-        new AzureKeyCredential(Environment.GetEnvironmentVariable("GITHUB_TOKEN")))
+        new AzureKeyCredential(Environment.GetEnvironmentVariable("GITHUB_TOKEN") ?? throw new InvalidOperationException("Missing GITHUB_TOKEN environment variable. Ensure you followed the instructions to setup a GitHub Token to use GitHub Models.")))
         .AsChatClient("Phi-3.5-MoE-instruct");
 
 var response = await client.CompleteAsync("What is AI?");