Prevent duplicate posts with better error message.

[mplorentz]

I have seen several users go through this flow:

1. They compose a new post or reply and hit publish
2. An error message is shown saying something about a SQL error
3. They don't see their post and their draft isn't cleared so they hit publish again
4. Eventually two copies of their post shows up
   What is happening here is that writing to the SSB log succeeded but copying the post into our SQLite database failed. In this case the post was essentially published to the SSB world correctly, but the user's local copy of Planetary won't show their post to them (usually only temporally). This is an unfortunate consequence of storing posts in two places. To mitigate this issue we should detect this case and show the user an error message that says something like "There was a problem displaying your post, but it was published successfully" and we should clear the draft.