Fix two translation string issues

The forms string is a duplicate of another string apart from the dot.

The regexp error text was a bit too techy.

Thanks to OdifYltsaeb for reporting.

oscar/apps/catalogue/abstract_models.py

@@ -709,7 +709,8 @@ class AbstractProductAttribute(models.Model):
         _('Code'), max_length=128,
         validators=[RegexValidator(
             regex=r'^[a-zA-Z_][0-9a-zA-Z_]*$',
-            message=_("Code must match ^[a-zA-Z_][0-9a-zA-Z_]*$"))])
+            message=_("Code can only contain the letters a-z, A-Z, digits "
+                      "and underscores, and can't start with a digit"))])
 
     TYPE_CHOICES = (
         ("text", _("Text")),

oscar/apps/customer/forms.py

@@ -194,7 +194,7 @@ class EmailUserCreationForm(forms.ModelForm):
         email = normalise_email(self.cleaned_data['email'])
         if User._default_manager.filter(email=email).exists():
             raise forms.ValidationError(
-                _("A user with that email address already exists."))
+                _("A user with that email address already exists"))
         return email
 
     def clean_password2(self):