This function works the same as 'openstorage' but will open a guild storage
window instead for the guild storage of the guild the invoking character belongs
to. This is a function because it returns a value - 0 if the guild storage was
opened successfully and 1 if it wasn't. (Notice, it's a ZERO upon success.)
Since guild storage is only accessible to one character at one time, it may fail
if another character is accessing the guild storage at the same time.

This will also fail and return 2 if the character does not belong to any guild.