JSON readtable error if github returns 403 forbidden

[bo-tato]

when url-user-agent is nil, dash-docs-install-docset gives the error:

dash-docs-read-json-from-url: JSON readtable error: 82

As github is returning 403 forbidden for requests that don't contain a user agent. Maybe in dash-docs-read-json-from-url we could locally set url-user-agent to 'default if it's nil. Or else return a more descriptive error message it fails to get the url with 403 forbidden rather than a json parse error.