Improve API return data for landing_zone_create with missing project iRODS collections

[mikkonie]

Reported by @Nicolai-vKuegelgen. Currently the landingzones API returns HTTP 400 if attempting to run landing_zone_create on a project for which iRODS collections have not been created.

The request itself is not technically invalid in this case. Hence, instead of 400 we should return 503 with a descriptive error message.

This is in line with the changes I recently made in #1847 and should be trivial to implement, so this would fit nicely in v1.0.0.

This is technically a breaking change, so I'll have to be sure to document it as such.

Tasks

• Update view to return 503 instead of 400
• Ensure descriptive error message
• Update tests
• Update documentation (including release notes)

[mikkonie]

Done. The message itself looked fine though.