Rename check_character_count method to check_is_message_to_long.

Add different error message for email and text if content is too long.
Use utils version with is_message_too_long method implemented for email templates.

app/notifications/validators.py

@@ -143,10 +143,19 @@ def check_if_service_can_send_to_number(service, number):
         return international_phone_info
 
 
-def check_content_char_count(template_with_content):
+def check_is_message_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 1000000 bytes. "
+                f"Your message is {template_with_content.content_size_in_bytes} bytes."
+            )
         raise BadRequestError(message=message)
 
 
@@ -175,7 +184,7 @@ def validate_template(template_id, personalisation, service, notification_type,
     # which means the length of the message can be exceeded because it's including the file.
     # The document download feature is only available through the api.
     if check_char_count:
-        check_content_char_count(template_with_content)
+        check_is_message_too_long(template_with_content)
 
     return template, template_with_content