Tailor message-too-long error message

depending on the notification type.

Up until now, only sms messages could get message-too-long error,
but now we also need to validate the size of email messages, so
the message content needs to be tailored to the notification type.

app/notifications/validators.py

@@ -144,10 +144,19 @@ def check_if_service_can_send_to_number(service, number):
         return international_phone_info
 
 
-def check_content_char_count(template_with_content):
+def check_message_is_not_too_long(template_with_content):
     if template_with_content.is_message_too_long():
-        message = f"Text messages cannot be longer than {SMS_CHAR_COUNT_LIMIT} characters. " \
-                  f"Your message is {template_with_content.content_count_without_prefix} characters"
+        message = "Your message is too long."
+        if template_with_content.template_type == SMS_TYPE:
+            message = (
+                f"Text messages cannot be longer than {SMS_CHAR_COUNT_LIMIT} characters. "
+                f"Your message is {template_with_content.content_count_without_prefix} characters long."
+            )
+        elif template_with_content.template_type == EMAIL_TYPE:
+            message = (
+                f"Emails cannot be longer than 7500000 bytes. "
+                f"Your message is {template_with_content.content_size_in_bytes} bytes."
+            )
         raise BadRequestError(message=message)
 
 
@@ -173,7 +182,7 @@ def validate_template(template_id, personalisation, service, notification_type):
 
     check_notification_content_is_not_empty(template_with_content)
 
-    check_content_char_count(template_with_content)
+    check_message_is_not_too_long(template_with_content)
 
     return template, template_with_content