handle error from custom loader

[n1ru4l]

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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