#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)
#6622 Logger refactoring: restored the "screen" backend to log to HTML notifications

The "screen" backend (WebNotificationHandler) now logs to HTML notification boxes using the `Notification` api.
I've re-set the default log level to WARN, and logged PHP notices/warnings to "warning" so that on a default install they are shown in the page in a yellow/warning notification box.

For the record, the default log level had previously been changed from WARN to ERROR because screen logging was previously messing the HTML/JSON output (which was breaking archiving). With the logger refactoring this is no longer a problem.
#6622 Logger refactoring: simplified LogTest

Cannot add tests for the screen backend because I would need to mock the session, which is not possible right now because it's all static stuff.
Merge pull request #6842 from piwik/log-refactoring-2

Separated error and exception handling from logging