change return type of page_subscriber() to void

We deliberately ignore errors from page_subscriber() so there is
no point in having a non-void return value.
Provide more context in the error message logged if paging failed.

Add a comment in an implementation override of base_grace_paging_request()
in the test suite to make return value semantics more clear.

Change-Id: Ie18c2ba53d2055d3eaff8c9ed939eb844af6dd2e
Related: I48f5efbcddd98e15256edfca06ba0ae6acb5bab1
2 files changed