WIP: token error handling (pending)

[Sundraiz-Shah]

What changed

Before this fix, when something went wrong (wrong token, expired token, wrong URL), the tool would silently fail with a generic or confusing message. Now it tells you exactly what the problem is.

Situation	Before	After
Wrong GitHub token	"nothing harvested" (no reason given)	"The provided GitHub token is invalid"
Expired GitHub token	"nothing harvested" (no reason given)	"The GitHub token has expired"
Wrong GitHub URL	"nothing harvested" (no reason given)	"GitHub repository not found"
Wrong GitLab token	"GitLab requires a valid personal access token"	"The provided GitLab token is invalid"
Expired GitLab token	"GitLab requires a valid personal access token"	"The GitLab token has expired"
Wrong GitLab URL	"GitLab requires a valid personal access token"	"GitLab repository not found"

In short: instead of a confusing dead end, you now get a clear message telling you what went wrong and where.