#3 Improved error messages

gms-ui/src/App.vue

gms-ui/src/api/server/index.js

gms/pom.xml

gms/src/main/java/it/inaf/ia2/gms/authn/SecurityConfig.java

...n/java/it/inaf/ia2/gms/controller/HomePageController.java

.../java/it/inaf/ia2/gms/controller/KeepAliveController.java

...n/java/it/inaf/ia2/gms/exception/BadRequestException.java

.../main/java/it/inaf/ia2/gms/exception/ErrorController.java

...src/main/java/it/inaf/ia2/gms/exception/GmsException.java

...ain/java/it/inaf/ia2/gms/exception/NotFoundException.java

...java/it/inaf/ia2/gms/exception/UnauthorizedException.java

...src/main/java/it/inaf/ia2/gms/persistence/LoggingDAO.java

gms/src/main/resources/application.properties

gms/src/main/resources/public/error/404.html

gms/src/main/resources/public/error/error.html