Remove generic exceptions. (#2913)

* Replace instances of Exception with better ones in reflection utils.

* Replace instances of Exception with better ones in the websocket project.

* Finish replacing generic exceptions.

* Tiny tweak to reflection utils for consistency with the .NET library.

experiment/Discord.Net.BuildOverrides/BuildOverrides.cs

@@ -263,7 +263,7 @@ namespace Discord
                 var result = await client.PostAsync($"{ApiUrl}/overrides/{id}/dependency", new StringContent($"{{ \"info\": \"{name}\"}}", Encoding.UTF8, "application/json"));
 
                 if (!result.IsSuccessStatusCode)
-                    throw new Exception("Failed to get dependency");
+                    throw new HttpRequestException("Failed to get dependency");
 
                 using (var ms = new MemoryStream())
                 {