Hosting or hostmaster should suppress error messages by default

Following up on #2821365: Clients cannot create sites because of missing database options, I felt is was rather strange that a client was able to view an error message. Since aegir offers an out-of-the-box experience (it works directly after installing), people might forget about switching error reporting off.

I think either hosting or hostmaster should set the error_level variable to ERROR_REPORT_HIDE during hook_install(). Probably requiring an update hook to set it for existing sites.