Use better exception handling

Parsing the message of an exception to decide what to do
next is a bad practice, because it's easy that the message
might be changed by mistake in the future. To enforce the
coupling in a stronger way it's better to use exceptions
of different type depending on the kind of error, so that
we rely on the compiler enforcing the behaviour when doing
changes in this error handling areas in the future.

This also makes the code a bit more simple and readable.

TLSharp.Core/Network/MtProtoSender.cs

@@ -279,9 +279,7 @@ namespace TLSharp.Core.Network
                 {
                     var resultString = Regex.Match(errorMessage, @"\d+").Value;
                     var dcIdx = int.Parse(resultString);
-                    var exception = new InvalidOperationException($"Your phone number registered to {dcIdx} dc. Please update settings. See https://github.com/sochix/TLSharp#i-get-an-error-migrate_x for details.");
+                    throw new MigrationNeededException(dcIdx);
-                    exception.Data.Add("dcId", dcIdx);
-                    throw exception;
                 }
                 else
                 {
@@ -484,4 +482,15 @@ namespace TLSharp.Core.Network
             return new MemoryStream(new byte[len], 0, len, true, true);
         }
     }
+
+    internal class MigrationNeededException : Exception
+    {
+        internal int DC { get; private set; }
+
+        internal MigrationNeededException(int dc)
+            : base ("$Your phone number is registered to a different dc: {dc}. Please migrate.")
+        {
+            DC = dc;
+        }
+    }
 }

TLSharp.Core/TelegramClient.cs

@@ -110,16 +110,9 @@ namespace TLSharp.Core
 
                     completed = true;
                 }
-                catch (InvalidOperationException ex)
+                catch (MigrationNeededException ex)
                 {
-                    if (ex.Message.StartsWith("Your phone number registered to") && ex.Data["dcId"] != null)
+                    await ReconnectToDcAsync(ex.DC);
-                    {
-                        await ReconnectToDcAsync((int)ex.Data["dcId"]);
-                    }
-                    else
-                    {
-                        throw;
-                    }
                 }
             }