Add additional sink potion #dipping effects

Also put "Nothing seems to happen." into c_common_strings, since it's
used all over several files.

src/potion.c

@@ -2405,7 +2405,7 @@ potion_dip(struct obj *obj, struct obj *potion)
                 prinv((char *) 0, obj, 0L);
                 return ECMD_TIME;
             } else {
-                pline("Nothing seems to happen.");
+                pline1(nothing_seems_to_happen);
                 goto poof;
             }
         }