Better error/guidance when GitHub authentication is not granted to the PR extension

[rebornix]

Testing this on a new environment and kept running into this. Later on realize that I didn't grant GitHub auth to GHPRI. The error message can be more descriptive.

## Response
~~~
ERROR while calling tool: Not authenticated
Please check your input and try again.
~~~

[joshspicer]

I've started improving error handling with #7114 but won't help with this case