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diagnostics: Add error codes for GHCi error conditions
GHCi has a bunch of error conditions which are just reported to the user by `hPutStrLn stderr` rather than anything structured. It would be good if the GHCi errors were made structured and given error codes like the errors in GHC. Related to !10305 which introduces the `GHCiMessage` wrapper but for the purpose of changing how the messages are printed.
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