Inform users that they can't send another introduction when still having one pending
To reproduce: send ~~as many introductions to your friend as you can.~~ 2 introductions to the same friend.
```
23:12:27.249 [pool-2-thread-3] WARN o.b.b.desktop.viewmodel.DbViewModel - Unhandled exception in database executor
org.briarproject.briar.api.client.ProtocolStateException: null
at org.briarproject.briar.introduction.IntroducerProtocolEngine.onRequestAction(IntroducerProtocolEngine.java:88)
at org.briarproject.briar.introduction.IntroductionManagerImpl.makeIntroduction(IntroductionManagerImpl.java:363)
at org.briarproject.briar.desktop.introduction.IntroductionViewModel$makeIntroduction$1.invoke(IntroductionViewModel.kt:109)
at org.briarproject.briar.desktop.introduction.IntroductionViewModel$makeIntroduction$1.invoke(IntroductionViewModel.kt:106)
at org.briarproject.briar.desktop.viewmodel.DbViewModel$runOnDbThread$1.invoke(DbViewModel.kt:47)
at org.briarproject.briar.desktop.viewmodel.DbViewModel$runOnDbThread$1.invoke(DbViewModel.kt:44)
at org.briarproject.briar.desktop.threading.BriarExecutorsImpl.onDbThread$lambda-0(BriarExecutorsImpl.kt:33)
at org.briarproject.bramble.TimeLoggingExecutor.lambda$execute$0(TimeLoggingExecutor.java:36)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
at java.base/java.lang.Thread.run(Thread.java:832)
```
issue