Improve error messages for invalid color input in resistor-color

[deekshahavaragi]

Currently, the resistor-color exercise raises a generic error (or unclear
exception) when an invalid color is provided.

It would be more helpful for learners if the exercise raised a ValueError
with a clear and descriptive error message, such as:

"Invalid color: pink"

This improves feedback for beginners without changing the exercise intent
or expected behavior.

Proposed change:

• Validate color input explicitly
• Raise ValueError with a meaningful message for invalid colors

[github-actions]

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