requirePackages gives potentially misleading error message

[mb706]

When requirePackages fails to load a package, it usually tells the user that the packages in question are not installed. This ignores the fact that packages may fail to load for other reasons (e.g. "too many DLLs loaded"). This is especially confusing when default.method = "load", since requireNamespace has quietly set to TRUE, which therefore suppresses all other info about why a package didn't load.