How to add an error message to script?

Question

Hi, every one
I got this script
function paste in place to next master
I want to add an error message to it, if there was no such thing as master B, C,D,E,F,G
instead of system error.
Teetan
var
  myDialog = app.dialogs.add({name:"Copy Selected item(s) to chosen Master",canCancel:true}),
  mStringList = ["Master A", "Master B","Master C", "Master D", "Master E","Master F", "Master G"];
with(myDialog){
  with(dialogColumns.add()){
  staticTexts.add({staticLabel:"Select a Master you want to copy to"});
  with(dialogRows.add()){
  var mySelection = dropdowns.add({stringList:mStringList, selectedIndex:0});
app.doScript('main()', ScriptLanguage.JAVASCRIPT, undefined, UndoModes.FAST_ENTIRE_SCRIPT, "Paste in place");
function main(){
  if (!app.documents.length || !app.selection.length)
  alert( "Nothing selected!" );
  exit();
  if (myDialog.show() == true)
  copy_to_next_master(); 
function copy_to_next_master() { 
var
  mChoice = mySelection.selectedIndex,
  mMsg = "Done.\rWhat you select is pasted in place to next documents - " + mStringList[mChoice],
  nSel = app.selection.length,
  mDone = false,
  i,n,nextDoc,curSel;   
for ( i = 1; i < app.documents.length; i++) {   
  nextDoc = app.documents[i];   
  targetMaster = nextDoc.masterSpreads[mChoice];
  if (!targetMaster.isValid) {
  mMsg += "\rCan't detect " + mStringList[mChoice] + " in a doc: " + nextDoc.name;
  continue;
  mDone = true;
  for ( n = 0; n < nSel; n++ ) { 
  curSel = app.selection[n]; 
  curSel.duplicate( nextDoc.masterSpreads[mChoice] ); 
mDone ? alert(mMsg) : alert("Nothing copied");

Answer 1

Sorry Jarek
I made a big mistake
I want add an error message in to this:
var curSel = app.selection[0]; 
for ( var i = 1; i < app.documents.length; i++) { 
    var nextDoc = app.documents[i]; 
    if (!curSel) { alert( "Nothing selected!" ); exit(); } 
    curSel.duplicate( nextDoc.masterSpreads[1] ); 
in case of I just want to paste in place to only on choice
Teetan