Keeps errors returned by libicu prepare (nodexmpp/node-stringprep#65)
Keeps errors returned by libicu prepare (nodexmpp/node-stringprep#65)

Only attempt to use the javascript fallback routines when the native
libicu function is not available. This fixes two issues:
1) Overwriting the libicu error with a success from the fallback
2) Reporting a LIBICU_NOT_AVAILABLE error when fallbacks are disabled
and libicu is present, but returns an error

20xr/node-stringprep,20xr/node-stringprep,20xr/node-stringprep