Added IGuild.IsOwner, cleaned a few exceptions

src/Discord.Net/Common/Entities/Guilds/IGuild.cs

@@ -11,6 +11,8 @@ namespace Discord
         int AFKTimeout { get; }
         /// <summary> Returns true if this guild is embeddable (e.g. widget) </summary>
         bool IsEmbeddable { get; }
+        /// <summary> Returns true if the current user owns this guild. </summary>
+        bool IsOwner { get; }
         /// <summary> Gets the name of this guild. </summary>
         string Name { get; }
         int VerificationLevel { get; }

src/Discord.Net/Rest/Entities/Guilds/Guild.cs

@@ -46,6 +46,8 @@ namespace Discord.Rest
         /// <inheritdoc />
         public DateTime CreatedAt => DateTimeHelper.FromSnowflake(Id);
         /// <inheritdoc />
+        public bool IsOwner => OwnerId == Discord.CurrentUser.Id;
+        /// <inheritdoc />
         public string IconUrl => API.CDN.GetGuildIconUrl(Id, _iconId);
         /// <inheritdoc />
         public string SplashUrl => API.CDN.GetGuildSplashUrl(Id, _splashId);
@@ -155,11 +157,15 @@ namespace Discord.Rest
         /// <inheritdoc />
         public async Task Leave()
         {
+            if (IsOwner)
+                throw new InvalidOperationException("Unable to leave a guild the current user owns.");
             await Discord.BaseClient.LeaveGuild(Id).ConfigureAwait(false);
         }
         /// <inheritdoc />
         public async Task Delete()
         {
+            if (!IsOwner)
+                throw new InvalidOperationException("Unable to delete a guild the current user does not own.");
             await Discord.BaseClient.DeleteGuild(Id).ConfigureAwait(false);
         }
 

src/Discord.Net/Rest/Entities/Guilds/UserGuild.cs

@@ -41,14 +41,14 @@ namespace Discord.Rest
         public async Task Leave()
         {
             if (IsOwner)
-                throw new InvalidOperationException("Unable to leave a guild the current user owns, use Delete() instead.");
+                throw new InvalidOperationException("Unable to leave a guild the current user owns.");
             await Discord.BaseClient.LeaveGuild(Id).ConfigureAwait(false);
         }
         /// <inheritdoc />
         public async Task Delete()
         {
             if (!IsOwner)
-                throw new InvalidOperationException("Unable to leave a guild the current user owns, use Delete() instead.");
+                throw new InvalidOperationException("Unable to delete a guild the current user does not own.");
             await Discord.BaseClient.DeleteGuild(Id).ConfigureAwait(false);
         }