✏️ Fix #260, #263, and #265

[HeySreelal]

Fix #260

The HttpClientException is now being exported with teledart.dart. This will let the users setup error handlers for the same.

[HeySreelal]

Just refactored the test/ directory as well. The test/TeleDart_test.dart wasn't following the dart conventions and Linter wasn't too happy about it.

I just refactored the linter issue by renaming and adding one test. Let's consider adding more tests in the future.

[HeySreelal]

Added one more commit, making the methods editMessageText, editMessageCaption, editMessageMedia, editMessageLiveLocation, stopMessageLiveLocation, and editMessageReplyMarkup to have the return type Future<dynamic> to be able to return either Message or bool as per the documentation. (Fixes #263).

[DinoLeung]

Thanks for your hard work @HeySreelal I'll be looking into this PR this weekend.

[HeySreelal]

Also added fix for #265. Made the replyMarkup parameter in the Telegram.stopPoll method to be nullable as per the documentation.