feat: verbose error for instance urls with api suffix

[Tymek]

About the changes

In context of Edge, instance URL is easy to mistake with SDK API URL. This could've save me couple minutes of confusion.

WARN schedule_revalidation_of_startup_tokens: unleash_edge::http::unleash_client: Failed to validate tokens. Requested url: [https://sandbox.getunleash.io/enterprise/api/edge/validate]. Got status: 401
INFO schedule_revalidation_of_startup_tokens: unleash_edge::http::unleash_client: Try passing the instance URL without '/api'.

Discussion points

• Do we want it as part of the warning instead?
• Do we expect tests for this change?

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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[chriswk]

I think I'm fine with no tests here. I like the log message.