Short-circuit if trying to export an empty channel or a channel with mismatching boundaries

Closes #1024

DiscordChatExporter.Cli/Commands/Base/ExportCommandBase.cs

@@ -169,10 +169,10 @@ public abstract class ExportCommandBase : DiscordCommandBase
             );
         }
 
+        // Export
         var cancellationToken = console.RegisterCancellationHandler();
         var errors = new ConcurrentDictionary<Channel, string>();
 
-        // Export
         await console.Output.WriteLineAsync($"Exporting {channels.Count} channel(s)...");
         await console.CreateProgressTicker().StartAsync(async progressContext =>
         {

DiscordChatExporter.Core/Discord/DiscordClient.cs

@@ -333,10 +333,9 @@ public class DiscordClient
         IProgress<Percentage>? progress = null,
         [EnumeratorCancellation] CancellationToken cancellationToken = default)
     {
-        // Get the last message in the specified range.
-        // This snapshots the boundaries, which means that messages posted after the export started
+        // Get the last message in the specified range, so we can later calculate progress based on its date.
+        // This also snapshots the boundaries, which means that messages posted after the export started
         // will not appear in the output.
-        // Additionally, it provides the date of the last message, which is used to calculate progress.
         var lastMessage = await TryGetLastMessageAsync(channelId, before, cancellationToken);
         if (lastMessage is null || lastMessage.Timestamp < after?.ToDate())
             yield break;

DiscordChatExporter.Core/Discord/Snowflake.cs

@@ -59,4 +59,8 @@ public partial record struct Snowflake : IComparable<Snowflake>, IComparable
 
         return Value.CompareTo(other.Value);
     }
+
+    public static bool operator >(Snowflake left, Snowflake right) => left.CompareTo(right) > 0;
+
+    public static bool operator <(Snowflake left, Snowflake right) => left.CompareTo(right) < 0;
 }

DiscordChatExporter.Core/Exporting/ChannelExporter.cs

@@ -18,13 +18,20 @@ public class ChannelExporter
         IProgress<Percentage>? progress = null,
         CancellationToken cancellationToken = default)
     {
+        // Check if the channel is empty
+        if (request.Channel.LastMessageId is null)
+            throw DiscordChatExporterException.ChannelIsEmpty();
+
+        // Check if the 'after' boundary is valid
+        if (request.After is not null && request.Channel.LastMessageId < request.After)
+            throw DiscordChatExporterException.ChannelIsEmpty();
+
         // Build context
         var context = new ExportContext(_discord, request);
         await context.PopulateChannelsAndRolesAsync(cancellationToken);
 
         // Export messages
         await using var messageExporter = new MessageExporter(context);
-
         await foreach (var message in _discord.GetMessagesAsync(
                            request.Channel.Id,
                            request.After,