Travis: Initiating build for commit '5c2669513e8561ce2f62aa08c37ef806e43001d7' on branch 'log-refactoring-2': #6622 Logger refactoring: Separated error and exception handling from logging

- the error handler logs warnings and notices, and turns errors into exceptions (`ErrorException`)
- the exception handler catches all uncatched exceptions and display them to the user (HTML or CLI)
- the "screen" logging backend has been removed
- I've normalized exceptions/errors shown to the user in HTML (wether they are catched by the FrontController or not)