notify user when asking for output from a dead process

src/main/java/JobServ/ProcessController.java

@@ -103,6 +103,10 @@ class ProcessController {
      * gets output from process
      */
     public String getOutput(int lines) {
+        if(this.killedManually) {
+            return "[-] SERVER: Process has already been killed by a JobServ client!";
+	}
+
         String output = "";
         for (int i = 0; i < lines; i++) {
             String newLine = null;