Infrastructure exceptions in TaskActivityExceptions are not properly serialized

As pointed out by @robs in [this Durable Functions issue](https://github.com/Azure/azure-functions-durable-extension/issues/697), this [catch block](https://github.com/Azure/durabletask/blob/main/src/DurableTask.Core/TaskActivityDispatcher.cs#L172) sets the details to be more of an error message rather than a serialized exception, resulting in a `TaskFailedExceptionDeserializationException` in the orchestrator function. This is very miselading.

Fixing this to properly serialize the original exception thrown by the infrastructure could debatably be considered a breaking change, as customers may be taking a dependency on `TaskFailedExceptionDeserializationException` being thrown, but it feels more like a bug fix.