Handling customizer failure better

[XhmikosR]

When an error occurs during the less build, we get a nice error and all, but in order for the user to try again, they have to ctrl+f5 the page to try again.

To reproduce, just remove a closing bracket of a less var and hit download.

/CC @mdo

[mdo]

How might we make this better? This is a longstanding issue—if we have no specific idea how to do this, I'd like to close it out.

[cvrebert]

Doing this is fairly straightforward. It's just that nobody has put in the time to code it up,

[mdo]

Closing and punting to v4.