Differentiate between various error conditions when calling Bmgr methods

Differentiate between
1. when the service is null
2. the service is not activated
3. there is a remote exception
4. we cant get a restore session from the service

Bug: 137156961

Test: atest -v CtsBackupTestCases CtsBackupHostTestCases
Test: atest -v BmgrTest

Change-Id: I63f1005f43d8e3e23bbd629b183f128fa02bbac1