mcp: add CallToolResult.getError (#481)

Provide a way for middleware to get the error from a tool call,
demonstrating that we can add this functionality without making a
breaking change.

Leave getError unexported for now; we can export it (and setError) at
any time.

For #64.

mcp/cmd_test.go

@@ -226,7 +226,7 @@ func TestCmdTransport(t *testing.T) {
 			&mcp.TextContent{Text: "Hi user"},
 		},
 	}
-	if diff := cmp.Diff(want, got); diff != "" {
+	if diff := cmp.Diff(want, got, ctrCmpOpts...); diff != "" {
 		t.Errorf("greet returned unexpected content (-want +got):\n%s", diff)
 	}
 	if err := session.Close(); err != nil {