more informative exception for unknown event types
more informative exception for unknown event types

Note: bear with me, this is all done in github editor.

I'm facing issue trying to retrieve events that I defined in a F# script (evaluating/running the script again results in potentially different assembly name):

```fsharp
let store = DocumentStore.For(fun o ->
  o.Connection(connectionString)
  o.Events.set_StreamIdentity Marten.Events.StreamIdentity.AsString
)
```

When I run this code:

```fsharp
let stream = session.Events.FetchStream streamId
```

I'm getting this exception, which doesn't help troubleshooting the origin of the issue:

```
System.ArgumentNullException: Value cannot be null.
Parameter name: key
   at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
   at Baseline.ConcurrentCache`2.Fill(TKey key, Func`2 onMissing)
   at Marten.Events.StringIdentifiedEventSelector.Resolve(DbDataReader reader, IIdentityMap map, QueryStatistics stats)
   at Marten.Linq.SelectorExtensions.Read[T](ISelector`1 selector, DbDataReader reader, IIdentityMap map, QueryStatistics stats)
   at Marten.Events.EventQueryHandler`1.Handle(DbDataReader reader, IIdentityMap map, QueryStatistics stats)
   at Marten.Services.CommandRunnerExtensions.<>c__DisplayClass2_0`1.<Fetch>b__0(NpgsqlCommand c)
   at Marten.Services.ManagedConnection.Execute[T](NpgsqlCommand cmd, Func`2 func)
   at Marten.Events.EventStore.FetchStream(String streamKey, Int32 version, Nullable`1 timestamp)
   at <StartupCode$FSI_0015>.$FSI_0015.main@()
```

If there is a way to override how the event type is retrieved at runtime, it would be a good idea to point to it in the exception message, but I'm just learning about Marten so haven't yet figured that out.

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