Fixed the rate limit issue for openai embedder

[mmassime]

Description

Previously there was no check to make sure that the openai key used when setting up an openai embedder was not expired. I added two checks:

1. When inserting the key: an error will be shown and the user will have to insert a new key.
2. Every time the cat is instantiated: this is done becuase the user can still insert a valid key which could expire in the following days and this could cause problems when the cat is launched. To avoid this, the check is re-executed at any instantiation of the cat and if a non-valid key is detected then the embedder is set to DumbEmbedder and a warning is logged in the console.

Related to issue #461

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

[pieroit]

Closing this to work on a more general, non openai-specific solution.
Thanks for this anyway :D