Toward #6: improve error message for missing extensions.

Created by: twomice

Regarding https://github.com/twomice/org.civicrm.extensionsui/issues/6#issuecomment-287804332: "The error message says the extension cannot be loaded from the following path but does not actually give the path."

This uses a kind of compromise language for missing extensions that should work acceptably whether the file path displays for not.