Fixed `ValueError` not raised (#5095)

Raised `ValueError` in `wagtail/images/utils.py` that was called but not raised

I work for Semmle and I noticed these issues using our LGTM code analyzer
https://lgtm.com/projects/g/wagtail/wagtail/alerts/?mode=tree&severity=error&ruleFocus=1505923886371

CONTRIBUTORS.rst

@@ -347,6 +347,7 @@ Contributors
 * Vladimir Knobel
 * Matt Collins
 * Thomas Elliott
+* damianosSemmle
 
 Translators
 ===========

wagtail/images/tests/test_image_operations.py

@@ -619,3 +619,11 @@ class TestBackgroundColorFilter(TestCase):
             file=get_test_image_file(),
         )
         self.assertRaises(ValueError, fil.run, image, BytesIO())
+
+    def test_invalid_length(self):
+        fil = Filter(spec='width-400|bgcolor-1234')
+        image = Image.objects.create(
+            title="Test image",
+            file=get_test_image_file(),
+        )
+        self.assertRaises(ValueError, fil.run, image, BytesIO())

wagtail/images/utils.py

@@ -57,6 +57,6 @@ def parse_color_string(color_string):
         g = int(color_string[2:4], 16)
         b = int(color_string[4:6], 16)
     else:
-        ValueError('Color string must be either 3 or 6 hexadecimal digits long')
+        raise ValueError('Color string must be either 3 or 6 hexadecimal digits long')
 
     return r, g, b