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 (closed) which introduces the GHCiMessage wrapper but for the purpose of changing how the messages are printed.

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