Allow poisonous vapours to target spaces that might have monsters (Si…

…egurt)

You can now cast this spell on any space, but if there is no monster there
it will fizzle, much as Airstrike does.

crawl-ref/source/spl-clouds.cc

@@ -127,15 +127,16 @@ spret_type cast_poisonous_vapours(int pow, const dist &beam, bool fail)
     }
 
     monster* mons = monster_at(beam.target);
-    if (!mons || mons->submerged() || !you.can_see(*mons))
+    if (!mons || mons->submerged())
     {
-        mpr("You can't see any monster there!");
-        return SPRET_ABORT;
+        fail_check();
+        canned_msg(MSG_SPELL_FIZZLES);
+        return SPRET_SUCCESS; // still losing a turn
     }
 
-    if (actor_cloud_immune(*mons, CLOUD_POISON))
+    if (actor_cloud_immune(*mons, CLOUD_POISON) && mons->observable())
     {
-        mprf("But poisonous clouds would do no harm to %s!",
+        mprf("But poisonous vapours would do no harm to %s!",
              mons->name(DESC_THE).c_str());
         return SPRET_ABORT;
     }