cmake-gui: Suppress incidental error messages while choosing build tree

Avoid printing

    Error: <path> is not a directory

to the console for every update to the GUI's build directory widget.

Source/cmake.cxx

@@ -4016,7 +4016,9 @@ bool cmake::Open(std::string const& dir, DryRun dryRun)
   this->SetHomeDirectory("");
   this->SetHomeOutputDirectory("");
   if (!cmSystemTools::FileIsDirectory(dir)) {
+    if (dryRun == DryRun::No) {
       std::cerr << "Error: " << dir << " is not a directory\n";
+    }
     return false;
   }