fix pull request #498 - unseen demon "looks angry"

When a potentially bribable demon lord becomes angry because the
hero is wielding Excalibur or Demonbane, avoid "It looks angry"
if the demon can't be seen.  The pull request just suppressed the
message in that situation; I've added an alternate one.

Fixes #498

doc/fixes37.0

@@ -491,6 +491,8 @@ some rolling boulder trap feedback was inconsistent
 change "killed by <a foo>, while {paralyzed|frozen} by <a foo>" into
 	"killed by <a foo>, while {paralyzed|frozen}" if the killer caused
 	hero's helplessness
+"It looks very angry" would be given if a hero wielding Excalibur or Demonbane
+	offended an unseen demon lord
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository

src/minion.c

@@ -225,7 +225,10 @@ demon_talk(register struct monst *mtmp)
 
     if (uwep && (uwep->oartifact == ART_EXCALIBUR
                  || uwep->oartifact == ART_DEMONBANE)) {
-        pline("%s looks very angry.", Amonnam(mtmp));
+        if (canspotmon(mtmp))
+            pline("%s looks very angry.", Amonnam(mtmp));
+        else
+            You_feel("tension building.");
         mtmp->mpeaceful = mtmp->mtame = 0;
         set_malign(mtmp);
         newsym(mtmp->mx, mtmp->my);