error when users put non-GSM chars in a sms template

additionally, this moves the formatted_list jinja macro into a python
function, so that it can be called from the form validator

app/main/validators.py

@@ -1,13 +1,15 @@
 from wtforms import ValidationError
 from notifications_utils.template import Template
+from notifications_utils.gsm import get_non_gsm_characters
+
+from app.main._blacklisted_passwords import blacklisted_passwords
 from app.utils import (
     Spreadsheet,
     is_gov_user
 )
-from ._blacklisted_passwords import blacklisted_passwords
 
 
-class Blacklist(object):
+class Blacklist:
     def __init__(self, message=None):
         if not message:
             message = 'Password is blacklisted.'
@@ -18,7 +20,7 @@ class Blacklist(object):
             raise ValidationError(self.message)
 
 
-class CsvFileValidator(object):
+class CsvFileValidator:
 
     def __init__(self, message='Not a csv file'):
         self.message = message
@@ -28,7 +30,7 @@ class CsvFileValidator(object):
             raise ValidationError("{} isn’t a spreadsheet that Notify can read".format(field.data.filename))
 
 
-class ValidGovEmail(object):
+class ValidGovEmail:
 
     def __call__(self, form, field):
         from flask import url_for
@@ -40,7 +42,7 @@ class ValidGovEmail(object):
             raise ValidationError(message)
 
 
-class NoCommasInPlaceHolders():
+class NoCommasInPlaceHolders:
 
     def __init__(self, message='You can’t have commas in your fields'):
         self.message = message
@@ -48,3 +50,12 @@ class NoCommasInPlaceHolders():
     def __call__(self, form, field):
         if ',' in ''.join(Template({'content': field.data}).placeholders):
             raise ValidationError(self.message)
+
+
+class OnlyGSMCharacters:
+    def __call__(self, form, field):
+        non_gsm_characters = get_non_gsm_characters(field.data)
+        if non_gsm_characters:
+            raise ValidationError('The following characters are not allowed in text messages: {}'.format(
+                ', '.join(non_gsm_characters)
+            ))