hdkeychain: update test case error string due to btcec changes

A bug was recently fixed in btcec wherein we would fail to detect
invalid point decompressions for the curve. This how now been fixed, and
as a result, we'll fail an invalid point earlier in the ParsePubKey
method. We update the error string to reflect this change.

hdkeychain/extendedkey_test.go

@@ -857,7 +857,7 @@ func TestErrors(t *testing.T) {
 		{
 			name: "pubkey not on curve",
 			key:  "xpub661MyMwAqRbcFtXgS5sYJABqqG9YLmC4Q1Rdap9gSE8NqtwybGhePY2gZ1hr9Rwbk95YadvBkQXxzHBSngB8ndpW6QH7zhhsXZ2jHyZqPjk",
-			err:  errors.New("pubkey isn't on secp256k1 curve"),
+			err:  errors.New("invalid square root"),
 		},
 		{
 			name:      "unsupported version",