Create specific error class to signal uplink fetcher errors

[nogates]

Hello there,

As described in #1439, we are suggesting here to use a specific class when throwing errors while fetching the schema from Apollo Uplink.

This should allow us to capture those errors using the specific class name, instead of relying on the actual message.

I've decided to transform all the errors in that file (since they are all related to the same process of fetching the schema). Also, please let me know if you think the error class should be placed somewhere else, so we don't need to export it up to the top

Thank you!

Fixes

[apollo-cla]

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[trevor-scheer]

This LGTM. Thanks for the changes! Can you please add an entry to the gateway-js/CHANGELOG.md? We'll port this to the version-0.x branch when this lands.

cc @cpeacock for awareness since you participated on the linked issue - sounds like there might be an Atlas issue we can close out when this lands?

[nogates]

This LGTM. Thanks for the changes! Can you please add an entry to the gateway-js/CHANGELOG.md? We'll port this to the version-0.x branch when this lands.

Done! :)

[cpeacock]

pinging @clenfest for awareness