Followup to r1481827: don't spam stderr by default for API consumers who are
not going to 'just abort()'.

This may introduce compiler warnings on windows, but those were deemed to be
acceptable.

Suggested by: rhuijben
Review by: rhuijben

* subversion/include/svn_error.h
  (svn_error_get_malfunction_handler): New API.

* subversion/libsvn_subr/error.c
  (svn_error_get_malfunction_handler): Implement it.

* subversion/libsvn_fs/fs-loader.c
  (default_warning_func):
    Only use stderr if the next line will abort().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481836 13f79535-47bb-0310-9956-ffa450edef68