Document the new error handling functionality from python-http-client

[thinkingserious]

Issue Summary

Here is a PR that implements the new error handling: sendgrid/python-http-client#17

Here is an example of usage:

import sendgrid
import os
from sendgrid.helpers.mail import *
from python_http_client import exceptions

sg = sendgrid.SendGridAPIClient(apikey=os.environ.get('SENDGRID_API_KEY'))
from_email = Email("dx@sendgrid.com")
to_email = Email("elmer.thomas@sendgrid.com")
subject = "Sending with SendGrid is Fun"
content = Content("text/plain", "and easy to do anywhere, even with Python")
mail = Mail(from_email, subject, to_email, content)
try:
    response = sg.client.mail.send.post(request_body=mail.get())
except exceptions.BadRequestsError as e:
    print(e.body)
    exit()
print(response.status_code)
print(response.body)
print(response.headers)

The examples should go under USE_CASES.md and be referenced from TROUBLESHOOTING.md under error handling.

[mptap]

@mbernier: PR: #481