add extended error code to exception

[kai5263499]

I needed the ability to determine when I should blacklist email addresses that were returning errors from SparkPost's REST API and I discovered that the extended error code was already being returned so this change is simply to add that extended error code to the exception message so that downstream consumers can parse it out with something like

p = re.compile(r'Code: (?P<code>\d+) ')
m = p.search(e_str)
codes = [int(c) for c in m.group('code')]

[coveralls]

Coverage remained the same at 100.0% when pulling 5057aa4 on kai5263499:master into b3262a5 on SparkPost:master.

[richleland]

Thanks for the contribution @kai5263499!