Include "actual size" in error message for content size check

[dylanbussone]

Problem:

The current error message for content size is merely

`content size at ${data.url} over limit: ${data.size}`

where data.size is the limit. It gives no indication of how much bigger the actual size was than the limit, which is valuable information.

Request:

Include the actual size in the error message in addition to the limit.
I have a local branch with this addition, if you give me permissions I'd be happy to open a PR.

[jimmywarting]

I'm having thoughts on deprecating/removing size option...