marginally better feature parsing

Fail slightly more gracefully when brackets supplied the wrong way
round.

pitest/src/main/java/org/pitest/plugin/FeatureParser.java

@@ -54,7 +54,11 @@ private void extractValue(String part, Map<String, List<String>> vals) {
       if (current == null) {
         current = new ArrayList<>();
       }
-      current.add(pairs[i + 1].trim());
+      if (i + 1 < pairs.length) {
+        current.add(pairs[i + 1].trim());
+      } else {
+        throw new RuntimeException("Could not parse feature. Parameters should be configured with +feature(param[value], param2[value2])");
+      }
      vals.put(key, current);
     }
   }

pitest/src/test/java/org/pitest/plugin/FeatureParserTest.java

@@ -1,6 +1,7 @@
   package org.pitest.plugin;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.AssertionsForClassTypes.assertThatCode;
 
 import java.util.Collections;
 import java.util.List;
@@ -11,17 +12,12 @@
 import java.util.Optional;
 
 public class FeatureParserTest {
-
-  @Rule
-  public ExpectedException thrown = ExpectedException.none();
-
   FeatureParser testee = new FeatureParser();
 
   @Test
   public void shouldRejectInputNotStartingWithPlusOrMinus() {
-    this.thrown.expect(RuntimeException.class);
-    final FeatureSetting actual = parse("FOO");
-    assertThat(actual.addsFeature()).isFalse();
+    assertThatCode( () -> parse("FOO"))
+            .hasMessageContaining("Could not parse FOO");
   }
 
   @Test
@@ -88,6 +84,12 @@ public void shouldParseListValues() {
     assertThat(actual.getList("things")).contains("1","2","3","4");
   }
 
+  @Test
+  public void failsCleanlyWhenBracketsWrongWayRound() {
+    assertThatCode(() -> parse("+BAR[things(1])]"))
+            .hasMessageContaining("Could not parse feature. Parameters should be configured with +feature(param[value], param2[value2])");
+  }
+
 
   private FeatureSetting parse(String dsl) {
     final List<FeatureSetting> actual = this.testee.parseFeatures(Collections.singletonList(dsl));