Support `error_messages` dict as argument to RegexBlock

The individual `error_message` kwarg on RegexField is deprecated in Django 1.8
(and removed in Django 1.10), so it's appropriate for RegexBlock to follow the
same convention.

docs/topics/streamfield.rst

@@ -122,7 +122,15 @@ RegexBlock
 
 ``wagtail.wagtailcore.blocks.RegexBlock``
 
-A single-line text input that validates a string against a regex expression. The regular expression used for validation must be supplied as the first argument, or as the keyword argument ``regex``. The message text used to indicate a validation error can be customised using the ``error_message`` keyword argument to pass a custom message. The keyword arguments ``regex``, ``required``, ``max_length``, ``min_length`` and ``error_message`` are accepted.
+A single-line text input that validates a string against a regex expression. The regular expression used for validation must be supplied as the first argument, or as the keyword argument ``regex``. To customise the message text used to indicate a validation error, pass a dictionary as the keyword argument ``error_messages`` containing either or both of the keys ``required`` (for the message shown on an empty value) or ``invalid`` (for the message shown on a non-matching value):
+
+.. code-block:: python
+
+    blocks.RegexBlock(regex=r'^[0-9]{3}$', error_message={
+        'invalid': "Not a valid library card number."
+    })
+
+The keyword arguments ``regex``, ``required``, ``max_length``, ``min_length`` and ``error_messages`` are accepted.
 
 URLBlock
 ~~~~~~~~

wagtail/wagtailcore/blocks/field_block.py

@@ -164,13 +164,13 @@ class DecimalBlock(FieldBlock):
 class RegexBlock(FieldBlock):
 
     def __init__(self, regex, required=True, max_length=None, min_length=None,
-                 error_message=None, *args, **kwargs):
+                 error_messages=None, *args, **kwargs):
         self.field = forms.RegexField(
             regex=regex,
             required=required,
             max_length=max_length,
             min_length=min_length,
-            error_message=error_message,
+            error_messages=error_messages,
         )
         super(RegexBlock, self).__init__(*args, **kwargs)
 

wagtail/wagtailcore/tests/test_blocks.py

@@ -274,22 +274,41 @@ class TestRegexBlock(TestCase):
     def test_raises_required_error(self):
         block = blocks.RegexBlock(regex=r'^[0-9]{3}$')
 
-        with self.assertRaises(ValidationError):
+        with self.assertRaises(ValidationError) as context:
             block.clean("")
 
+        self.assertIn('This field is required.', context.exception.messages)
+
+    def test_raises_custom_required_error(self):
+        test_message = 'Oops, you missed a bit.'
+        block = blocks.RegexBlock(regex=r'^[0-9]{3}$', error_messages={
+            'required': test_message,
+        })
+
+        with self.assertRaises(ValidationError) as context:
+            block.clean("")
+
+        self.assertIn(test_message, context.exception.messages)
+
     def test_raises_validation_error(self):
         block = blocks.RegexBlock(regex=r'^[0-9]{3}$')
 
-        with self.assertRaises(ValidationError):
+        with self.assertRaises(ValidationError) as context:
             block.clean("[/]")
 
+        self.assertIn('Enter a valid value.', context.exception.messages)
+
     def test_raises_custom_error_message(self):
         test_message = 'Not a valid library card number.'
-        block = blocks.RegexBlock(regex=r'^[0-9]{3}$', error_message=test_message)
+        block = blocks.RegexBlock(regex=r'^[0-9]{3}$', error_messages={
+            'invalid': test_message
+        })
 
-        with self.assertRaises(ValidationError):
+        with self.assertRaises(ValidationError) as context:
             block.clean("[/]")
 
+        self.assertIn(test_message, context.exception.messages)
+
         html = block.render_form(
             "[/]",
             errors=ErrorList([ValidationError(test_message)]))