Throw AvaException if auth error

[skylord123]

It may be good to add in the functionality for AvaException to be thrown on authentication errors. This way I can catch the exception and handle accordingly (notify me if something is wrong).

Things to accomplish this:

• Add constant to AvaException to help filter by this exception type when caught (https://github.com/skylord123/AvaTax-Calc-REST-PHP/blob/a55f5bcc59cf2e8d0c7f20dc9d61a4c8e7725d9a/src/AvaTax/AvaException.php#L7-L10)
• Check for message with source Avalara.Web.Authorization and if found throw AvaException with the new auth error code.