Fix some errors which didn't trigger a break loop anymore

Specifically use ErrorMayQuit instead of ErrorQuit in CheckIsPossList and
CheckIsDenseList to ensure this produces a break loop again:

    f:=function()
    local a,v;
      a:=[1.2,3];
      v:=2;
      v:=a{v};
    end;
    f();

Fixes gap-system#3011

src/error.c

@@ -548,7 +548,7 @@ void ErrorMayQuit(const Char * msg, Int arg1, Int arg2)
 void CheckIsPossList(const Char * desc, Obj poss)
 {
  if ( ! IS_POSS_LIST( poss ) ) {
- ErrorQuit("%s: <positions> must be a dense list of positive integers",
+ ErrorMayQuit("%s: <positions> must be a dense list of positive integers",
  (Int)desc, 0 );
  }
 }
@@ -560,7 +560,7 @@ void CheckIsPossList(const Char * desc, Obj poss)
 void CheckIsDenseList(const Char * desc, const Char * listName, Obj list)
 {
  if (!IS_DENSE_LIST(list)) {
- ErrorQuit("%s: <%s> must be a dense list", (Int)desc, (Int)listName);
+ ErrorMayQuit("%s: <%s> must be a dense list", (Int)desc, (Int)listName);
  }
 }