Return correct MySQL error code when inserting into nonexistent columns.

[nicktobey]

Fixes #2344

This PR also adds a test framework for testing MySQL error codes, since this doesn't appear to currently be tested. This should make adding tests for other error codes easy.

Some of the MySQL error codes that I expected us to use (such as 1050: ERTableExists) have zero usages in GMS or Dolt. We're probably returning 1105: ERUnknownError for these.

[zachmu]

Test looks good, thanks for adding.

Main comment is that any user-facing error should be in the sql package. Probably all the error types you moved are in fact user-facing and should be moved there. We tend to put them all in errors.go. There might be a couple internal errors in there (namely "not supported" type errors that should be caught in analysis and never make it to plan time), and these should probably live where you found them. The rest should go in sql/errors.go.

[nicktobey]

Test looks good, thanks for adding.

Main comment is that any user-facing error should be in the sql package. Probably all the error types you moved are in fact user-facing and should be moved there. We tend to put them all in errors.go. There might be a couple internal errors in there (namely "not supported" type errors that should be caught in analysis and never make it to plan time), and these should probably live where you found them. The rest should go in sql/errors.go.

Makes sense. I'll undo the package refactor and just move errors from their current packages to sql as we discover that they're user-facing.