feat: Expose more detail in dev server errors [FUN-701]

[mike-zorn]

Expose more details about dev server errors like what thing wasn't found. Requested in #520.

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Related Jira issue: FUN-701: Merge customer PR (error fidelity)

[Copilot]

Pull Request Overview

This PR enhances dev server error responses by exposing detailed error messages and refactoring error handling throughout the codebase. Key updates include:

• Switching from errors.Is to errors.As for more expressive error type handling.
• Updating error messages to reflect actual error details and replacing legacy error variables with newly defined error types.
• Adjusting tests to assert errors via errors.As instead of errors.Is.

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/dev_server/sdk/store_facade.go	Updated WriteError to use errors.As and print the error’s own message.
internal/dev_server/model/sync_test.go	Changed test assertions from ErrorIs to ErrorAs for project insertion.
internal/dev_server/model/sync.go	Refactored error check for project creation to use errors.As.
internal/dev_server/model/store.go	Removed legacy error variable definitions for NotFound and AlreadyExists.
internal/dev_server/model/override_test.go	Adjusted tests to assert ErrNotFound errors using errors.As.
internal/dev_server/model/override.go	Modified to return NewErrNotFound with detailed error information.
internal/dev_server/model/error.go	Removed legacy custom error implementation.
internal/dev_server/model/err_not_found.go	Added new error type for not found errors with detailed message formatting.
internal/dev_server/model/err_already_exists.go	Added new error type for already exists errors (error message needs update).
internal/dev_server/db/sqlite_test.go	Updated tests to verify errors using errors.As.
internal/dev_server/db/sqlite.go	Updated error returns to use NewErrNotFound and NewErrAlreadyExists.
internal/dev_server/api/put_override_flag.go	Adjusted error check to use errors.As for identifying ErrNotFound.
internal/dev_server/api/post_add_project.go	Refactored error handling to use errors.As and return err.Error() message.
internal/dev_server/api/delete_flag_override.go	Changed error handling to use errors.As for ErrNotFound errors.