Improve RelaceEditTool error handling for file not found errors

[codegen-sh]

Description

This PR improves the error handling in the RelaceEditTool when a file is not found. Instead of raising a FileNotFoundError exception, it now returns a RelaceEditObservation with an error status and a helpful message.

Changes

• Modified the relace_edit function to return a RelaceEditObservation with an error status when a file is not found
• Added a more descriptive error message that includes the full filepath and suggests using the full filepath relative to the repository root

Why

Previously, when a user provided an invalid filepath to the RelaceEditTool, it would raise a FileNotFoundError exception that wasn't properly passed back to the assistant as an observation. This change ensures that file not found errors are handled consistently with other errors and returned as observations, allowing the assistant to handle them gracefully.

Fixes the issue where users would see errors like:

FileNotFoundError('File not found: codegen_agi.py')

instead of getting a helpful error message as part of the tool's observation.

[CLAassistant]

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