fix(gateway): ipld.ErrNotFound should result in a 404

In case of blockstore restriction like with --offline or similar restriction, returning a 500 is inapropriate.

It's also going back to a previous gateway behavior (at least in kubo v0.22, possibly later).

gateway/errors.go

@@ -12,6 +12,7 @@ import (
 	"github.com/ipfs/boxo/gateway/assets"
 	"github.com/ipfs/boxo/path/resolver"
 	"github.com/ipfs/go-cid"
+	ipld "github.com/ipfs/go-ipld-format"
 	"github.com/ipld/go-ipld-prime/datamodel"
 )
 
@@ -188,6 +189,10 @@ func isErrNotFound(err error) bool {
 		return true
 	}
 
+	if ipld.IsNotFound(err) {
+		return true
+	}
+
 	// Checks if err is of a type that does not implement the .Is interface and
 	// cannot be directly compared to. Therefore, errors.Is cannot be used.
 	for {

gateway/gateway_test.go

@@ -846,7 +846,7 @@ func TestErrorBubblingFromBackend(t *testing.T) {
 		})
 	}
 
-	testError("500 Not Found from IPLD", &ipld.ErrNotFound{}, http.StatusInternalServerError)
+	testError("404 Not Found from IPLD", &ipld.ErrNotFound{}, http.StatusNotFound)
 	testError("404 Not Found from path resolver", &resolver.ErrNoLink{}, http.StatusNotFound)
 	testError("502 Bad Gateway", ErrBadGateway, http.StatusBadGateway)
 	testError("504 Gateway Timeout", ErrGatewayTimeout, http.StatusGatewayTimeout)