Initialize even if related table is not available to user

[bstroebl]

In rare occasions a user has access to a layer A and can load it in QGIS but has no access rights to a layer B which is related to A.
In that case initializing fails because B cannot be loaded into the project.

Proposed behaviour:

1. do not show an error (maybe an info is more adequate)
2. omit the widget accessing B in A's mask