NoSuchGroupException toast is shown when private group is removed
* Android version: 4.4.2
* Phone model: LGE LG-D150 (w35_global_com)
* Briar version: 1.3.4 (76b7e6f)
* User feedback: "NoSuchGroupException"
Log:
```
05-23 22:32:59.452 I/GroupListViewModel: Private group removed
05-23 22:32:59.453 I/ThreadListViewModel: Group removed
05-23 22:32:59.456 I/BaseActivity: Pausing GroupActivity
05-23 22:32:59.467 I/BaseActivity: Starting NavDrawerActivity
05-23 22:32:59.467 I/BaseActivity: Resuming NavDrawerActivity
05-23 22:32:59.950 I/BaseActivity: Stopping GroupActivity
05-23 22:32:59.954 I/BaseActivity: Destroying GroupActivity
05-23 22:32:59.967 W/DbViewModel: org.briarproject.bramble.api.db.NoSuchGroupException
org.briarproject.bramble.api.db.NoSuchGroupException
at org.briarproject.bramble.db.DatabaseComponentImpl.mergeGroupMetadata(DatabaseComponentImpl.java:784)
at org.briarproject.bramble.client.ClientHelperImpl.mergeGroupMetadata(ClientHelperImpl.java:224)
at org.briarproject.bramble.client.ClientHelperImpl.lambda$mergeGroupMetadata$7(ClientHelperImpl.java:218)
at org.briarproject.bramble.client.ClientHelperImpl.lambda$mergeGroupMetadata$7$ClientHelperImpl(ClientHelperImpl.java)
at org.briarproject.bramble.client.-$$Lambda$ClientHelperImpl$7us-EF4vPLR_8c0LCNvGmtb6d4I.run(lambda)
at org.briarproject.bramble.db.DatabaseComponentImpl.transaction(DatabaseComponentImpl.java:200)
at org.briarproject.bramble.client.ClientHelperImpl.mergeGroupMetadata(ClientHelperImpl.java:218)
at org.briarproject.briar.client.MessageTrackerImpl.storeMessageId(MessageTrackerImpl.java:92)
at org.briarproject.briar.android.threaded.ThreadListViewModel.lambda$storeMessageId$1(ThreadListViewModel.java:222)
at org.briarproject.briar.android.threaded.ThreadListViewModel.lambda$storeMessageId$1$ThreadListViewModel(ThreadListViewModel.java)
at org.briarproject.briar.android.threaded.-$$Lambda$ThreadListViewModel$8pSkGFRn7z9NtDqesywkHNLVqgk.run(lambda)
at org.briarproject.briar.android.viewmodel.DbViewModel.lambda$runOnDbThread$0(DbViewModel.java:77)
at org.briarproject.briar.android.viewmodel.DbViewModel.lambda$runOnDbThread$0$DbViewModel(DbViewModel.java)
at org.briarproject.briar.android.viewmodel.-$$Lambda$DbViewModel$WFnJ2C0vmjF_vFVS_futFODEXKA.run(lambda)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
```
What seems to be happening is that ThreadListViewModel is asynchronously storing the list position, and it gets a NoSuchGroupException because the group has recently been removed. The exception is harmless and should probably be logged rather than shown in the UI.
issue