Give the error message we use for composing multiple error chains a specific
number to allow api users to separate the message chains.

* subversion/include/svn_error_codes.h
  (SVN_ERR_COMPOSED_ERROR): New error code.

* subversion/libsvn_subr/error.c
  (svn_error_compose_create): Wrap using the SVN_ERR_COMPOSED_ERROR error.
  (svn_error_root_cause): Add commented code to explain the problem with
    this function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553266 13f79535-47bb-0310-9956-ffa450edef68