Rewrite the “usage” example for the greater good

Since this came up on #haskell today:

    (03:35:07 PM) danilo2: Hello! I just dioscovered that lenses are PURE
    EVIL. I spent hours to track a bug caused by how lenses are stupid. I
    would love to ask you If uis there **any way** to fix the following
    behaviour. I've got a simple code: http://lpaste.net/112230 After
    refactoring I changed some names and everythiong compiled and worked even
    if the name was not found in the datatype. Instead of reporting error,
    lenses used the monoid to insert empty v

    (03:35:31 PM) danilo2: Is there any way, to make lenses throwing error
    when field name is not matched instead of just hiding this from me and
    using monoid there?