LegacyRouter is no longer used for routing, only in existing plugins to match the _PAGE_ parameter.
So we change a few of its values there, to match the new ones defined in TemplatePage.
@see discussion in shaarli/Shaarli#1537
If a PHP is raised while executing plugin hook, Shaarli will display an error instead of rendering the error page (or just ending in fatal error for default hooks).
Also added phpErrorHandler which is handled differently that regular errorHandler by Slim.:
With the new routes, all pages are not all at the same folder level anymore
(e.g. /shaare and /shaare/123), so we can't just use './' everywhere.
The most consistent way to handle this is to prefix all path with the proper variable,
and handle the actual path in controllers.
