Make some error messages more user-friendly

[Moez Bouhlel]

some error messages looks unclear and not user-friendly and needs a new design and some times needs othes buttons (to easily solve the error).
for ex, "renaming label" error message when the fs (vfat) is mounted is not enough simple, also, i think it will be better if it also has "unmount & continue" button that will helps user to solve the error with easier way
as i'm not developer, i just made a simple mock-up about how it could be: ( http://ubuntuone.com/3OfUP6pFCGVxivahfYZNPD )
also, i think if a simple message (information message) that tells user to unmount the device will be better than error message