Authentication errors are being thrown as Server errors

[jmnsf]

When Sendgrid returns 403 bad user/pass you'd expect a SendgridTookit::AuthenticationFailed error but get a SendgirdToolkit::SendgridServerError instead:

pry(SendGridExporter)> e.class
=> SendgridToolkit::SendgridServerError
pry(SendGridExporter)> e.message
=> "The sengrid server returned an error. #<HTTParty::Response:0x1064a2638 parsed_response={\"error\"=>\"Bad username / password\"}, @response=#<Net::HTTPForbidden 403 Forbidden readbody=true>, @headers={\"server\"=>[\"nginx/1.4.4\"], \"date\"=>[\"Tue, 09 Sep 2014 09:01:26 GMT\"], \"content-type\"=>[\"text/html\"], \"transfer-encoding\"=>[\"chunked\"], \"connection\"=>[\"close\"]}>"