Throw a slightly more friendly message when a student opens an empty file

[ausbin]

This error gets propagated to autograders, so a more friendly error message is useful

The current behavior is this, which confuses students:

java.lang.NullPointerException: Cannot read field "circuits" because "circuitFile" is null
	at com.ra4king.circuitsim.gui.CircuitSim.lambda$loadCircuits$57(CircuitSim.java:1477)
	at java.base/java.lang.Thread.run(Thread.java:833)