FCPPluginMessageEncodeDecodeTest: Fix failure

Fixes:

[junit] java.lang.AssertionError: errorCode should be alpha-numeric
[junit]     at freenet.pluginmanager.FredPluginFCPMessageHandler
           $FCPPluginMessage.<init>(FredPluginFCPMessageHandler.java:208)
[junit]     at freenet.pluginmanager.FredPluginFCPMessageHandler
           $FCPPluginMessage.constructReplyMessage(
           FredPluginFCPMessageHandler.java:304)
[junit]     at freenet.pluginmanager.FredPluginFCPMessageHandler
           $FCPPluginMessage.constructErrorReply(
           FredPluginFCPMessageHandler.java:336)
[junit]     at freenet.node.fcp.FCPPluginMessageEncodeDecodeTest
           .testEncodeDecode(FCPPluginMessageEncodeDecodeTest.java:48)