fix: handle service messages and unsupported message types

[EzekialSA]

This is a fix for the --skip-same implementation when attempting downloads of unsupported message types.

• Add UnsupportedMessageTypeError to handle MessageService, MessageEmpty, etc.
• Update GetSingleMessage to return specific error instead of generic 'invalid message'
• Add error handling in download iterator to skip system messages gracefully
• System messages include 'user joined', 'message pinned', etc. which aren't downloadable

[iyear]

Is this PR the same as #1114 and #1090? They have already been merged.