Add a message for the case of files which can not be deleted because they where referenced by others.

A player referenced in a unit can not be deleted without deleting the unit. A Unit used in a booklet can not be deleted without deleting the booklet. A booklet used in a
Testtakers-File can not be deleted without that.

most of them occured because of missing dependencies in the testingModule, some where console-outputs from backends.services (which are mocked now)

upgrades eslint ruleset to version 0.0.9 and removes `eslint-disable-next-line no-param-reassign`-exception where it it is possible

better names for `[GET] /workspace/ws_id/files`'s return type's properties.. `size` instead of `filesize` etc.

add info field to GetFileResponseData interface and display the the additional useful data it can contain

added ApiError instead of number as error result of all backend.services; added throwError to cancel loading of units

first successful run after merging tc + admin; workspace monitor extracted from admin; testcontroller works; syscheck does not work; admin login failed

* problem: interceptor in workspace module ignored!

* response type json removed
* logout removed
* optical markers deleted