[ticket/12362] Throw exception in schema generator on unresolvable dependency

Make sure we throw an exception in the schema generator if we come across an
unresolvable dependency. Otherwise we'll get stuck in a infinite loop that
needs to be cancelled by the user or the maximum execution time.

PHPBB3-12362
[ticket/12362] Add tests for schema generator

PHPBB3-12362
[ticket/12362] Do not use database test case as it's not needed

PHPBB3-12362
Merge remote-tracking branch 'Marc/ticket/12362' into develop-ascraeus

* Marc/ticket/12362:
  [ticket/12362] Do not use database test case as it's not needed
  [ticket/12362] Add tests for schema generator
  [ticket/12362] Throw exception in schema generator on unresolvable dependency