Tweaked error strings and slightly improved the appearance of the errors in the UI.

Made some of the error strings a little more user-friendly. In the UI, boldfaced the succeeded/failed message, and put the error message on the next line.

I think the design could be much improved in other ways (the fonts seem too small, centering doesn't really seem ideal, etc.), but I'm going to file that as a separate issue for Garth to look at since he's rethinking modals anyway.