feat: bulletproof error handling

[rosnovsky]

Make sure that whatever error happens in the package, it's clearly communicated to the user of the package.

• Issue with mention formatting (missing/incorrect domain, invalid postId)
• Issue with fetching post data (no response from the instance, error response, unexpected response or unexpected response format, etc)
• Issue with formatting the embedded card (in theory, if the process gets this far, cards should be generated successfully, but still need to explain what went wrong if there is an error)
• Unable to handle attachment format (which format it is and why it can't be handled)
• ???