Followup to r1572916, fix error return handling.

* subversion/include/private/svn_magic.h
* subversion/libsvn_subr/magic.c
  (svn_magic__init): Change return type.

* subversion/libsvn_client/add.c
  (add): Handle svn_magic__init return.

* subversion/libsvn_client/import.c
  (import): Handle svn_magic__init return.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1572921 13f79535-47bb-0310-9956-ffa450edef68