Throw exception on nonexistent font

In K, this used to throw RuntimeException.  Before this patch, NULL
was returned and no exceptions thrown.  With this patch,
FileNotFoundException is thrown.  This might be considered API
change and be undesirable as callers need to either catch it or
declare it to be thrown.

Bug: 16180181
Change-Id: If83d76857f640c6293a3d5d08fb89ceddbaf41bc