Just a note that in almost all cases the best and easiest way to check for an error in a 'then do' section of a block is to simply test for 'if error'. That is:
That 'error' value will be a string describing the error.
Unfortunately the Local Storage component is the one case where we don’t (yet) provide an 'error' block. We do, however, return the string “null” when there is no value from the Get block for the given key. The choice of returning a string, rather than a real null value (i.e. the “green null block”) was probably a bad choice on our part 
-Mark