Throw not found exception on unknown instance

[bobvandevijver]

Current behaviour is a PHP warning as url input is not correctly checkec, which results in an error page during development, but a white 200 OK response on production.

When catching this early with a NotFoundException, it is clear that the instance does not exist.

This change adds two major improvements:

• During development there will be a Symfony error page with a "Instance not found" instead of a "Notice: Undefined index: ".
• The user on production will be presented with an accurate 404 not found page, which can be customized by the application (instead of a 200 OK page with some random content, but white result).

Q	A
Bug fix?	yes
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets
License	MIT

Note, this is the rebased version of #303, as I got no response there and the source has changed.

[helios-ag]

Thanks