Create mockups for all popup dialogue (error / information / confirmation).
Also do we want animation when the popup close? Which one (example)?
If in the same time you can create a mockup with the loading icons. (like the upload)

Previously it was:

Messages should appear centered

I think that the error and informational messages should probably appear centered within the dialogue.

It may also be prudent to move the spinner to the end of the text rather than have it at the beginning, or to have it at the end of the title, or on a new line beneath the message.