MAGE-1030: Fix client creation

[damcou]

This change includes:

• Removal of client creation in the constructor to avoid such issues in the future
• Prevented unhandled crash on category merchandising when no credentials are configured at Default level
• Feedback to the user is now handled by the AlgoliaCredentialsManager class

[cammonro]

It's good we caught this scenario. Definitely don't want exceptions preventing injection of the helper class. However, I am concerned about the approach of not throwing exceptions at all.

[cammonro]

Looks good!