graphql: Recover from panic within goroutines used for resolving custom fields.

[pawanrawal]

We recover from any panics encountered within go-routines used for resolving custom fields and return the error back to the client. @MichaelJCompton couldn't find an easy way to add a test for this. I did test this by adding a panic to makeRequest and then running an e2e to verify all errors are returned properly. Resolves GRAPHQL-409.

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This change is 

[MichaelJCompton]

I also don't see an easy way to test with the way it's structured at the moment. However, we test the pattern pretty nicely elsewhere, so if it did turn out to be a problem here we could restructure.

One thing that has dropped off the list is that we aren't adding the GraphQL error paths in a few places now. I've added a ticket to backlog to see if we can add those where missing.

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @manishrjain and @pawanrawal)

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graphql/resolve/resolver.go, line 668 at r1 (raw file):

func internalServerError(err error, f schema.Field) *x.GqlError {
	return x.GqlErrorf("Evaluation of custom field failed because of an error: %s for field: %s"+

should we be wrapping the original error here - GQLWrapLocationf ? So it'll say it like

"evaluation of custom field f on type T failed because ..."