dftm commander, output a warning if the tool is missing

xref #125

src/u_dfmt.pas

@@ -308,7 +308,10 @@ begin
   if fDoc.isNotAssigned then
     exit;
   if not exeInSysPath('dfmt') then
+  begin
+    getMessageDisplay().message('DFMT is missing. See https://github.com/dlang-community/dfmt for more instructions on how to get DFMT.', nil, TAppMessageCtxt.amcAll, TAppMessageKind.amkWarn);
     exit;
+  end;
 
   fBackup.Assign(fDoc.Lines);
   prc := TProcess.create(nil);