Don't make callers responsible for losestr death

Remove callers' responsibility to deal with possible hero death when
calling losestr.  This is less fragile and error-prone than leaving it
in the caller's hands, but it means that death from the monster spell
'weaken target' no longer goes through done_in_by, and the death reason
is no longer "killed by <monster name>".

src/fountain.c

@@ -292,7 +292,7 @@ drinkfountain(void)
                 losehp(rnd(4), "unrefrigerated sip of juice", KILLED_BY_AN);
                 break;
             }
-            losestr(rn1(4, 3));
+            losestr(rn1(4, 3), "contaminated water", KILLED_BY);
             losehp(rnd(10), "contaminated water", KILLED_BY);
             exercise(A_CON, FALSE);
             break;