Add exchange health checks

[sjanel]

Sometimes exchanges are not reachable (maintenance for instance), or api key is not authorized for private requests (invalid IP for instance).

Would be good to add a health check in each ExchangePublic and ExchangePrivate to:

• Disable unit tests if exchange health check is red (in case of maintenance for instance).
• Automatically exclude exchanges in requests when list is not specified
• Improve errors diagnostics