Annotate http status assertion

[jeroen]

See #277. Adds option to annotate the stop_for_status error message with an assertion. This can be helpful for http client packages to provide more informative error feedback to the user.

req <- GET("https://httpbin.org/status/404")
stop_for_status(req)
## Error: client error: (404) Not Found

# More informative error
stop_for_status(req, "find spreadsheet")
## Error: find spreadsheet failed: (HTTP 404)

[hadley]

I think I'd prefer the errors to be a bit more parallel:

stop_for_status(req)
## Error: Not found (HTTP 404)

stop_for_status(req, "Find spreadsheet failed")
## Error: Find spreadsheet failed (HTTP 404)

But I can take it from here