Rename potentially misleading FS index related error codes.

Because the term "item index" has a specific meaning in logical addressing,
we should not reuse it for general FS index-related errors.

* subversion/include/svn_error_codes.h
  (SVN_ERR_FS_ITEM_INDEX_CORRUPTION,
   SVN_ERR_FS_ITEM_INDEX_REVISION,
   SVN_ERR_FS_ITEM_INDEX_OVERFLOW,
   SVN_ERR_FS_ITEM_INDEX_INCONSISTENT): Rename these error codes to ...
  (SVN_ERR_FS_INDEX_CORRUPTION,
   SVN_ERR_FS_INDEX_REVISION,
   SVN_ERR_FS_INDEX_OVERFLOW,
   SVN_ERR_FS_INDEX_INCONSISTENT): ... these.

* subversion/libsvn_fs_fs/index.c
* subversion/libsvn_fs_fs/transaction.c
* subversion/libsvn_fs_fs/verify.c
* subversion/libsvn_fs_x/index.c
* subversion/libsvn_fs_x/transaction.c
* subversion/libsvn_fs_x/verify.c
  (): Global search of the old error codes and replace them with the new ones.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1628158 13f79535-47bb-0310-9956-ffa450edef68