Add an option to allow freeze to continue when a 404 error is returned

	FREEZER_IGNORE_404_NOT_FOUND

docs/index.rst

@@ -274,6 +274,16 @@ are accepted:
 
     .. versionadded:: 0.12
 
+``FREEZER_IGNORE_404_NOT_FOUND``
+    If set to ``True`` (defaults False), Frozen-Flask won't stop freezing when
+    404 error is returned by your application.
+    In this case, a warning will be printed on stdout and the static page will
+    be generated using your 404 error page handler or flask's default one.
+    This can be usefull during development phase if you have already referenced
+    pages wich aren't written yet.
+
+    .. versionadded:: 0.12
+
 .. _mime-types:
 
 Filenames and MIME types

flask_frozen/__init__.py

@@ -52,6 +52,8 @@ class MissingURLGeneratorWarning(Warning):
 class MimetypeMismatchWarning(Warning):
     pass
 
+class NotFoundWarning(Warning):
+    pass
 
 class Freezer(object):
     """
@@ -100,6 +102,7 @@ def init_app(self, app):
                                   'application/octet-stream')
             app.config.setdefault('FREEZER_IGNORE_MIMETYPE_WARNINGS', False)
             app.config.setdefault('FREEZER_RELATIVE_URLS', False)
+            app.config.setdefault('FREEZER_IGNORE_404_NOT_FOUND', False)
 
     def register_generator(self, function):
         """Register a function as an URL generator.
@@ -259,9 +262,17 @@ def _build_one(self, url):
 
         # The client follows redirects by itself
         # Any other status code is probably an error
+        # except we explictly want 404 errors to be skipped
+        # (eg. while application is in development)
+        ignore_404 = self.app.config['FREEZER_IGNORE_404_NOT_FOUND']
         if response.status_code != 200:
-            raise ValueError('Unexpected status %r on URL %s' \
-                % (response.status, url))
+            if response.status_code == 404 and ignore_404:
+                warnings.warn('Ignored %r on URL %s' % (response.status, url),
+                              NotFoundWarning,
+                              stacklevel=3)
+            else:
+                raise ValueError('Unexpected status %r on URL %s' \
+                    % (response.status, url))
 
         destination_path = self.urlpath_to_filepath(url)
         filename = os.path.join(self.root, *destination_path.split('/'))

flask_frozen/tests.py

@@ -21,7 +21,7 @@
 from warnings import catch_warnings
 
 from flask_frozen import (Freezer, walk_directory,
-    MissingURLGeneratorWarning, MimetypeMismatchWarning)
+    MissingURLGeneratorWarning, MimetypeMismatchWarning, NotFoundWarning)
 from flask_frozen import test_app
 
 try:
@@ -168,6 +168,15 @@ def make_app(self):
             self.do_extra_config(app, freezer)
             yield temp, app, freezer
 
+    @contextmanager
+    def make_app_with_404(self):
+        # Build an app with a link to a non-existent route
+        with self.make_app() as (temp, app, freezer):
+            @freezer.register_generator
+            def non_existent_url():
+                yield '/404/'
+            yield temp, app, freezer
+
     @contextmanager
     def built_app(self):
         with self.make_app() as (temp, app, freezer):
@@ -271,6 +280,29 @@ def external_url():
                 else:
                     assert False, 'Expected ValueError'
 
+    def test_error_on_internal_404(self):
+        with self.make_app_with_404() as (temp, app, freezer):
+            # Test standard behaviour with 404 errors (freeze failure)
+            try:
+                freezer.freeze()
+            except ValueError as e:
+                error_msg = "Unexpected status '404 NOT FOUND' on URL /404/"
+                assert error_msg in e.args[0]
+            else:
+                assert False, 'Expected ValueError'
+
+    def test_warn_on_internal_404(self):
+        with self.make_app_with_404() as (temp, app, freezer):
+            # Enable 404 erros ignoring
+            app.config['FREEZER_IGNORE_404_NOT_FOUND'] = True
+            # Test warning with 404 errors when we choose to ignore them
+            with catch_warnings(record=True) as logged_warnings:
+                warnings.simplefilter("always")
+                freezer.freeze()
+                self.assertEqual(len(logged_warnings), 1)
+                self.assertEqual(logged_warnings[0].category,
+                                  NotFoundWarning)
+
     def test_warn_on_missing_generator(self):
         with self.make_app() as (temp, app, freezer):
             # Add a new endpoint without URL generator