Improve error message on regex()

Author: Edmundo Alvarez

src/main/java/org/graylog/plugins/pipelineprocessor/functions/strings/RegexMatch.java

@@ -52,7 +52,8 @@ public RegexMatchResult evaluate(FunctionArgs args, EvaluationContext context) {
         final Pattern regex = pattern.required(args, context);
         final String value = this.value.required(args, context);
         if (regex == null || value == null) {
-            throw new IllegalArgumentException();
+            final String nullArgument = regex == null ? "pattern" : "value";
+            throw new IllegalArgumentException("Argument '" + nullArgument + "' cannot be 'null'");
         }
         //noinspection unchecked
         final List<String> groupNames =