colorpalette: Adds a nice error message to something that caused me trouble

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@1723 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/colorpalette/colorpalette.pas

@@ -342,6 +342,9 @@ var
   end;
   
 begin
+  if not FileExists(FileName) then
+    raise Exception.Create(Format('[TCustomColorPalette.LoadPalette] File not found: %s', [FileName]));
+
   AssignFile(F, FileName);
   try
     Reset(F);