Error Handling: Manage unsupported providers

[nishika26]

Is your feature request related to a problem?
An unhandled exception occurs when an unsupported provider is used, causing confusion for users. It's important to manage unsupported providers gracefully to improve user experience.

Describe the solution you'd like

• Implement error handling for unsupported providers.
• Return a user-friendly message indicating supported providers instead of a traceback.

Original issue

Describe the bug

  File "app/core/providers.py", line 55, in validate_provider
    return Provider(provider.lower())

ValueError: Unsupported provider: soemthing. Supported providers are: openai, aws, langfuse, google, sarvamai, elevenlabs```
 when credential provider is not from the supported ones

**Expected behavior**
A clear and concise description of what you expected to happen.


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