Check for unknown Qortal-only AT function codes & add safety to AT running in general

src/main/java/org/qortal/at/AT.java

@@ -100,7 +100,11 @@ public class AT {
 
 		// [Re]create AT machine state using AT state data or from scratch as applicable
 		MachineState state = MachineState.fromBytes(api, loggerFactory, latestAtStateData.getStateData(), codeBytes);
-		state.execute();
+		try {
+			state.execute();
+		} catch (Exception e) {
+			throw new DataException(String.format("Uncaught exception while running AT '%s'", atAddress), e);
+		}
 
 		long creation = this.atData.getCreation();
 		byte[] stateData = state.toBytes();

src/main/java/org/qortal/at/QortalATAPI.java

@@ -449,6 +449,9 @@ public class QortalATAPI extends API {
 	public void platformSpecificPostCheckExecute(FunctionData functionData, MachineState state, short rawFunctionCode) throws ExecutionException {
 		QortalFunctionCode qortalFunctionCode = QortalFunctionCode.valueOf(rawFunctionCode);
 
+		if (qortalFunctionCode == null)
+			throw new IllegalFunctionCodeException("Unknown Qortal function code 0x" + String.format("%04x", rawFunctionCode) + " encountered");
+
 		qortalFunctionCode.execute(functionData, state, rawFunctionCode);
 	}