fix text-239

Author: inponomarev

src/main/java/org/apache/commons/text/TextStringBuilder.java

@@ -483,7 +483,7 @@ public TextStringBuilder append(final char[] chars, final int startIndex, final
             return appendNull();
         }
         if (startIndex < 0 || startIndex > chars.length) {
-            throw new StringIndexOutOfBoundsException("Invalid startIndex: " + length);
+            throw new StringIndexOutOfBoundsException("Invalid startIndex: " + startIndex);
         }
         if (length < 0 || startIndex + length > chars.length) {
             throw new StringIndexOutOfBoundsException("Invalid length: " + length);

src/test/java/org/apache/commons/text/TextStringBuilderTest.java

@@ -2393,4 +2393,21 @@ void testWrap_CharArray_Int_Exceptions() {
         assertThrows(IllegalArgumentException.class, () -> TextStringBuilder.wrap(ArrayUtils.EMPTY_CHAR_ARRAY, 1));
     }
 
+    @Test
+    void testErrorMessageShowsCorrectVariable() {
+        TextStringBuilder sb = new TextStringBuilder("Hello");
+        char[] chars = {'a', 'b', 'c'};
+
+        StringIndexOutOfBoundsException ex = assertThrows(
+                StringIndexOutOfBoundsException.class,
+                () -> sb.append(chars, 1, 4)
+        );
+        assertTrue(ex.getMessage().contains("length: 4"));
+
+        ex = assertThrows(
+                StringIndexOutOfBoundsException.class,
+                () -> sb.append(chars, 7, 3)
+        );
+        assertTrue(ex.getMessage().contains("startIndex: 7"));
+    }
 }