svn_error_symbolic_name

Prompted by Julian's review of this freshly-minted public API on IRC:

1. Question to API consumers (eg, client developers): would you find
this API useful? For reference, the current docstring is attached.

2. Should we print the symbolic name in release builds? For example:

    % $svn upgrade /
    svn: E155019 (WC_INVALID_OP_ON_CWD): Can't upgrade '/' as it is not a working copy root
    svn: E000002: Working copy database '/.svn/wc.db' not found
    svn: E000002: Additional errors:
    svn: E000002: Can't open file '/.svn/entries': No such file or directory
    zsh: exit 1 $svn upgrade /
    ### (in maintainer mode, this would show
    ### subversion/libsvn_wc/wc_db.c:15006: (apr_err=SVN_ERR_WC_INVALID_OP_ON_CWD)
    ### as well.)

3. Should the API return the error code, or the error code without the
"SVN_ERR_" prefix? I think stripping the error code is better because
then users who search for the error code will be more likely to reach
the users@ archive (with helpful posts) than commits@ archive (which
are unlikely to contain the information they seek). Julian disagrees,
but I'll let him present his own case :-)

I also attached a patch which implements (2), in case we want it.
(It'll be trivial to modify it to suit whichever route we pick for (2)
and (3).)