Replace sys.exit() with UnknownFamily exception

The Family loader has a default enabled 'fatal' parameter which
invokes sys.exit() if the family name is not recognised.
Replace this will a new exception UnknownFamily which is a subclass
of new exception SiteDefinitionError (alias NoSuchSite).

Change-Id: I27727a86207c8e6639945c64871393bc254a11c2
Merge "Replace sys.exit() with UnknownFamily exception"