Add new error.number attribute to error reports

[hymerman]

This starts at 1 and increments with each new report sent.

This is incredibly useful for several types of query, but particularly:

• Filtering to just the first error in each session, since often subsequent errors are knock-ons from the first
• Sorting errors in a filtered session on the order they happened, which isn't possible when errors happen rapidly enough to get the same timestamps

We have used this change in production on two projects and found it to be very useful.

[konraddysput]

Hey @hymerman - you can use application.session attribute to get all errors generated in one single session and order them by the timestamp (error creation time). Backtrace can do all of this by you. In your case, if you want to have this attribute, wouldn't it be better to use beforeSend method to add it?

[hymerman]

Thanks Konrad - so ordering by timestamp within a session is nearly perfect for that use case, but I did find that sometimes, multiple errors would get the timestamp because of how close together they were.

The main benefit from tracking an error order number though, is in being able to filter to the first error in a session. Quite often, one thing will go wrong, which will put the app in a strange state and then cause many further errors, which normally can't ever happen. Fixing the first error fixes all subsequent errors. So it's usually useful, when dealing with a storm of new errors after a release, to focus first on fixing the errors with "error.number == 1" :)

I logged a support request quite a while back to request this (here), but there wasn't any built-in way to do this at the time, which is why I added error.number