abi/bind: fix error-handling in generated wrappers for functions returning structs

[connorwstein]

In this PR the template was changed which ended up producing code like:

	var out []interface{}
	err := _NonExistentStruct.contract.Call(opts, &out, "Struct")

	outstruct := new(struct {
		A *big.Int
		B *big.Int
	})

	outstruct.A = out[0].(*big.Int)
	outstruct.B = out[1].(*big.Int)

	return *outstruct, err

which will panic instead of returning an error if the contract doesn't exist. I moved the error handling up before the indexing to avoid that (analogous to how non-struct responses are handled)

[MariusVanDerWijden]

LGTM, thanks for noticing, and adding a test 👍

[connorwstein]

@MariusVanDerWijden thanks for the quick review. The build failure looks like a flake - would you be able to re-run? Don't think I have permissions to do that.

[MariusVanDerWijden]

restarted

[holiman]

Thanks!