[PATCH - v3] readable error messages on non-ASCII systems

This version hits error.c only, no new APIs. Sending as an attachment per
http://subversion.apache.org/docs/community-guide/general.html#patches

[[[ Produce readable error messages on non-ASCII systems.

* subversion/libsvn_subr/error.c:
      print_error(): convert err->message and the message prefix to UTF-8
before calling svn_cmdline_printf.
      svn_error_wrap_apr(): remove UTF-8 conversion of apr_strerror output
before storing it in err->message to avoid a double conversion
]]]

Greg