CMake: explain better why PythonQt isn't found

CMakeModules/FindPythonQt.cmake

@@ -139,4 +139,16 @@ if(PYTHONQT_INCLUDE_DIR AND PYTHONQT_LIBRARY AND PYTHONQT_QTALL_LIBRARY)
   set(PYTHONQT_FOUND 1)
   set(PythonQt_FOUND ${PYTHONQT_FOUND})
   set(PYTHONQT_LIBRARIES ${PYTHONQT_LIBRARY} ${PYTHONQT_LIBUTIL} ${PYTHONQT_QTALL_LIBRARY})
+elseif(NOT PythonQt_FIND_QUIETLY)
+  set(_missing "")
+  if (NOT PYTHONQT_INCLUDE_DIR)
+    list(APPEND _missing "includes")
+  endif()
+  if (NOT PYTHONQT_LIBRARY)
+    list(APPEND _missing "library")
+  endif()
+  if (NOT PYTHONQT_QTALL_LIBRARY)
+    list(APPEND _missing "qtall")
+  endif()
+  message(STATUS "PythonQt not found, missing components ${_missing}")
 endif()