* NotFound Errors are now handled by API

Can replace the following SlapTool calls:
* softwareReleaseError
* availableSoftwareRelease
* buildingSoftwareRelease
* destroyedSoftwareRelease

See merge request nexedi/slapos.core!455

See merge request nexedi/slapos.core!478

See merge request nexedi/slapos.core!495

This is no longer guessed on Zope4

this way we can just assert the full page content

this does not depend on the details of the URL

See merge request nexedi/slapos.core!493