Give better error message for blacklisted password

Telling the user what to do, rather than the mistake they’ve made is
usually better.

app/main/forms.py

@@ -73,7 +73,7 @@ def password(label='Password'):
     return PasswordField(label,
                          validators=[DataRequired(message='Can’t be empty'),
                                      Length(8, 255, message='Must be at least 8 characters'),
-                                     Blacklist(message='That password is blacklisted, too common')])
+                                     Blacklist(message='Choose a password that’s harder to guess')])
 
 
 def sms_code():

tests/app/main/test_validators.py

@@ -17,7 +17,7 @@ def test_should_raise_validation_error_for_password(app_, mock_get_user_by_email
         form.password.data = password
 
         form.validate()
-        assert 'That password is blacklisted, too common' in form.errors['password']
+        assert 'Choose a password that’s harder to guess' in form.errors['password']
 
 
 def test_valid_email_not_in_valid_domains(app_):