By default, we abort / malfunction if some cache access function returned
an error such like out-of-range.  Usually, the server would plug its own
logging callback here.

During verification, however, there is no such server callback.  Moreover,
we want all errors to be reported nicely instead of aborting the process.

* subversion/libsvn_fs/fs-loader.c
  (verify_fs_warning_func):  New, empty warning callback
  (svn_fs_verify):  Let all errors be reported through the svn_error_t*
                    return value mechanism.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1622955 13f79535-47bb-0310-9956-ffa450edef68