COMP: Fix catch by value warning in ctkDICOMAppWidget
COMP: Fix catch by value warning in ctkDICOMAppWidget

```cpp
[374/516] Building CXX object Libs/DICOM/Widgets/CMakeFiles/CTKDICOMWidgets.dir/ctkDICOMBrowser.cpp.o
Libs/DICOM/Widgets/ctkDICOMAppWidget.cpp: In member function ‘void ctkDICOMAppWidget::setDatabaseDirectory(const QString&)’:
Libs/DICOM/Widgets/ctkDICOMAppWidget.cpp:407:25: warning: catching polymorphic type ‘class std::exception’ by value [-Wcatch-value=]
   catch (std::exception e)
```

SINTEFMedtek/CTK,SINTEFMedtek/CTK,commontk/CTK,SINTEFMedtek/CTK,SINTEFMedtek/CTK,commontk/CTK,SINTEFMedtek/CTK,commontk/CTK,commontk/CTK