Set registers via :let

This has a number of interesting uses, such as `let @+=@%`, which copies the currently filename to the system clipboard rather succinctly. Notably, the display version of let does not work with register names (e.g.  `let @+ @%`).
Is there a better pre-canned error message I can use here? Guess I should take a look at `Resources`... "Error" is rather a copout :)