feat(exceptions): separate failed signin error

[jorwoods]

Closes #1472

This makes sign in failures their own class of exceptions, while still inheriting from NotSignedInException to not break backwards compatability for any existing client code. This should allow users to get out more specific exceptions more easily on what failed with their authentication request.

[bcantoni]

@jorwoods do you think we should/could incorporate a change here for the minor issue #1450? I don't know whether that other issue is big enough to fix, but since your change here is in a related area maybe we could do together.

[jorwoods]

@bcantoni I made a tweak to how the ServerInfoItem does its error handling to raise the error immediately. It currently raises a NonXMLResponseError. Not sure if you would want something custom raised there to indicate that the problem is more extensive beyond it not being valid XML.

[jorwoods]

I also changed the log level on those messages from info to exception so that they'll bubble up more clearly in logs.

[bcantoni]

Did some quick testing and this change looks good.

Sign in with incorrect PAT, before:

tableauserverclient.server.endpoint.exceptions.NotSignedInError: (b'<?xml version=\'1.0\' encoding=\'UTF-8\'?><tsResponse xmlns="http://tableau.com/api" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://tableau.com/api https://help.tableau.com/samples/en-us/rest_api/ts-api_3_25.xsd"><error code="401001"><summary>Signin Error</summary><detail>The personal access token you provided is invalid.</detail></error></tsResponse>', 'https://MYSERVER/api/3.25/auth/signin')

Sign in with incorrect PAT, after:

tableauserverclient.server.endpoint.exceptions.FailedSignInError: Failed Sign In Error:

	401001: Signin Error
		The personal access token you provided is invalid.

[bcantoni]

On second thought I don't think we need to do anything specific for 1450 (no specific error for pointing to a non-Tableau server).

[jorwoods]

Do you want it to raise the NonXMLResponseError? Or should I revert that change?