Provide an informative error message if something goes wrong in setfont
Merge pull request #3545 from mdehoon/setfont_informative_error_message

MNT : Provide an informative error message if something goes wrong in setfont
DOC: fixed references from qt4 to qt5 in test_backend_qt5
Merge pull request #3556 from cimarronm/test_backend_qt5_doc_fixes

MNT : copy/paste corrections in test_backend_qt5