better error message when using deathcause on vermin

[myk002]

right now it's a stack trace' should be an explanatory message

[xBlackTalonX]

Much better than a stack trace! More minor feedback:
A) Selecting dead vermin within a dump of body parts (unit gui open) and running deathcause results in,
"Automatically chose first corpse at selected location. The horse died."

B) Placing the keyboard cursor on that same stack of body parts and running deathcause results in,
"deathcause: Please select a corpse"

C) Clicking the stack of body parts beings up a UI screen "item list" (I don't know if there is a common reference to this screen) and then running deathcause (with "item list" gui open) results in,
"deathcause: Please select a corpse"

D) Selecting the horse hair item in this list (item gui open),
"The horse died"

E) Selecting an item (toy boat) (item gui open),
"deathcause: Please select a corpse"

F) Selecting a living creature (pig) (unit gui open),
"The pig is not dead yet!"

G) Selecting dead vermin (rat) isolated on a tile (item gui open),
"deathcause: Please select a corpse"

H) Selecting a living creature standing on top of a pile of body parts and vermin (unit gui open),
"[unit] is not dead yet!"

I) Selecting an item (toy boat)(item gui open) in a stack of body parts and living creatures,
"Automatically chose first corpse at selected location. The horse died."

So ideally,

• B and C should have the result of A: stack of items selected with cursor or gui item list would report that the first corpse in the stack was selected. (or output a line for each item in the stack)
• A (and presumably G) would have the result of D: dead vermin 'remains' would just say, "The [vermin] died," since no further information is really needed to have that limited detail.
• Alternatively, vermin could trigger a message like F, "[vermin] cause of death unsupported" but then that message would seemingly also be appropriate in cases like D (the horse hair).

But this is all really minor stuff so probably not worth more time?

[myk002]

done in DFHack/scripts#1095