users: Fix checking restricted usernames

When editing an existing user, error is being thrown due to restricted
usernames check.  This is due to the username matching existing
username.

Also:

- Raise the validation error on the field instead of the entire form.

- Send error code along with validation error message.

- End the validation error message with a full stop for consistency.

CHANGELOG.md

@@ -1,6 +1,10 @@
 # Change Log
 All notable changes to this project will be documented in this file.
 
+## [Unreleased]
+### Fixed
+- users: Fixed checking restricted usernames.
+
 ## [0.10.0] - 2016-08-12
 ### Added
 - Added Disks module to show free space of mounted partitions and
@@ -83,6 +87,7 @@ All notable changes to this project will be documented in this file.
 - Fixed issue that could allow someone to start a module setup process
   without being logged in to Plinth.
 
+[Unreleased]: https://github.com/freedombox/Plinth/compare/v0.10.0...HEAD
 [0.10.0]: https://github.com/freedombox/Plinth/compare/v0.9.4...v0.10.0
 [0.9.4]: https://github.com/freedombox/Plinth/compare/v0.9.3...v0.9.4
 [0.9.3]: https://github.com/freedombox/Plinth/compare/v0.9.2...v0.9.3

plinth/modules/users/forms.py

@@ -52,12 +52,15 @@ GROUP_CHOICES = (
 
 class ValidNewUsernameCheckMixin(object):
     """Mixin to check if a username is valid for created new user."""
-    def clean(self):
+    def clean_username(self):
         """Check for username collisions with system users."""
-        if not self.is_valid_new_username():
-            raise ValidationError(_('Username is taken or is reserved'))
+        username = self.cleaned_data['username']
+        if self.instance.username != username and \
+           not self.is_valid_new_username():
+            raise ValidationError(_('Username is taken or is reserved.'),
+                                  code='invalid')
 
-        return super().clean()
+        return username
 
     def is_valid_new_username(self):
         """Check for username collisions with system users."""