Throw an explicit ImsException if the ImsService isnt available

In the case that the ImsService is not active for the subId requested,
do not return false because this can cause services to think that
a capability is not capable/available for a specific subId when in
reality, we haven't loaded the configs for it yet.

Bug: 218893458
Test: ImsCommonTests
Change-Id: I97d6304216ec19fcd45f9552eb4e19061f1a323a