Hovertext for dropped tx state

**What problem are you trying to solve?**
Recently, I had a transaction enter status "dropped." It took me a long time to figure out what that meant (as in, why did it enter that status?). I found the answer [here](https://github.com/MetaMask/Design/issues/25#issuecomment-404677979):

>Dropped – the tx nonce was already used

Thanks to @bdresser for writing that comment! Once I knew this was all it meant, debugging the cause of that became a great deal easier, and the workaround (resetting the account) also became easier.

**Describe the solution you'd like**
Hovertext on the status "Dropped" which at least contains the contents of the quote above, and possibly also ". Verify that you are using the expected account. Resetting the account in Settings->Advanced may help." so that users can figure out what to do from there.

Some of the other statuses could probably use hovertext too.

Start by locating the transaction status UI and the rendering path for the "Dropped" status in the extension. Add hovertext explaining that the transaction nonce was already used and the suggested account check or reset, then verify the text appears when that status is shown.

Written by the indexing model from the issue text.