Create DM: now any userId can be entered, so deal with the case of the userId does not exists.

Use same string resource value than Riot-Web

vector/src/main/java/im/vector/riotx/features/createdirect/CreateDirectRoomActivity.kt

@@ -28,6 +28,7 @@ import com.airbnb.mvrx.Fail
 import com.airbnb.mvrx.Loading
 import com.airbnb.mvrx.Success
 import com.airbnb.mvrx.viewModel
+import im.vector.matrix.android.api.failure.Failure
 import im.vector.matrix.android.api.session.room.failure.CreateRoomFailure
 import im.vector.riotx.R
 import im.vector.riotx.core.di.ScreenComponent
@@ -37,6 +38,7 @@ import im.vector.riotx.core.extensions.addFragmentToBackstack
 import im.vector.riotx.core.platform.SimpleFragmentActivity
 import im.vector.riotx.core.platform.WaitingViewData
 import kotlinx.android.synthetic.main.activity.*
+import java.net.HttpURLConnection
 import javax.inject.Inject
 
 class CreateDirectRoomActivity : SimpleFragmentActivity() {
@@ -91,8 +93,14 @@ class CreateDirectRoomActivity : SimpleFragmentActivity() {
         if (error is CreateRoomFailure.CreatedWithTimeout) {
             finish()
         } else {
+            val message = if (error is Failure.ServerError && error.httpCode == HttpURLConnection.HTTP_INTERNAL_ERROR /*500*/) {
+                // This error happen if the invited userId does not exist.
+                getString(R.string.create_room_dm_failure)
+            } else {
+                errorFormatter.toHumanReadable(error)
+            }
             AlertDialog.Builder(this)
-                    .setMessage(errorFormatter.toHumanReadable(error))
+                    .setMessage(message)
                     .setPositiveButton(R.string.ok, null)
                     .show()
         }

vector/src/main/res/values/strings_riotX.xml

@@ -29,4 +29,5 @@
 
     <!-- END Strings added by Others -->
 
+    <string name="create_room_dm_failure">"We couldn't create your DM. Please check the users you want to invite and try again."</string>
 </resources>