handle error from custom loader

Currently, an error for a custom loader is swallowed, leading to a bad DX:

https://github.com/ardatan/graphql-tools/blob/53292e675ec6ec11f5c7b9d244898340cf7e109e/packages/load/src/utils/custom-loader.ts#L18

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originally raised in https://github.com/dotansimha/graphql-code-generator/discussions/7928#discussioncomment-3285607

Start at packages/load/src/utils/custom-loader.ts around line 18 and trace how errors from a custom loader are handled. Review the linked discussion for the reported behavior; done means the custom-loader error is surfaced instead of swallowed, with the existing package behavior still working.

Written by the indexing model from the issue text.