An exception occurred in the last command executed; this reports that to
the subclass of SerializedExecutor.
The default implementation of this method throws an exception, which is
considered an error (see below). Implementors have two alternatives:
Arrange not to throw from your commands' run(), or if they do, provide an
override of this method that handles any exception that is thrown.
It is an error for this to throw an exception, and doing so will terminate
the thread with an IllegalStateException. Derivers must handle any reported
exceptions in other ways.