Improve error messages for mssing fields

Currently, if you have a query containing a missing field you get this sort of error:

{
	"message": "Internal error: `(Internal error: `Couldn't get field named \"repo_name\" from schema.`., split_id: 01GTFCVJXZZS6HCQQGGR9KXXNG), (Internal error: `Couldn't get field named \"repo_name\" from schema.`., split_id: 01GTFCVX897SH37QZDPCV6ZY0Q), (Internal error: `Couldn't get field named \"repo_name\" from schema.`., split_id: 01GTFCXDJ2Y1T03FN9QN6Q0H46), (Internal error: `Couldn't get field named \"repo_name\" from schema.`., split_id: 01GTFCZ8BN97C3ZQW9PSX8NA82), (Internal error: `Couldn't get field named \"repo_name\" from schema.`., split_id: 01GTFD138HV6HE8Y1CT572EC9X)`."
}

This is fine when you only have a couple of splits as I do here to some extent, but when you have more splits, the error gets a bit more spammy, it's also probably best for the user to abstract away the duplicates in the event that it's the same error.

No files, tests, or entry points are named. Start by reproducing a query with a missing field and trace where the schema error is assembled across splits; done means duplicate missing-field errors are abstracted into a concise user-facing message.