[SQLObject] fetchAlternateID()

Not sure if i just saw a message about this, but can't find it..

modification to _SO_fetchAlternateID to deal with the situation where
there is no record with that value.

    def _SO_fetchAlternateID(cls, dbIDName, value):
        result = cls._connection._SO_selectOneAlt(
            cls,
            [cls._idName] +
            [col.dbName for col in cls._columns],
            dbIDName,
            value)
        if not result:
            raise KeyError, "The object %s by the alternate ID (%s) %s
does not exist" % (cls.__name__, dbIDName, value)
        obj = cls(result[0])
     ....

Now, this raise the question of what error to raise. We're doing a
similar check in _init() if the ID is non-existent, and KeyError
isn't exactly the right error to throw for these "no such object
exists". We're catching a generic LookupError for now for both these
cases, but I suggesting an SQLObject-specific lookup error:

class SQLObjectLookupError(LookupError):
     pass

(or maybe named NoSuchObject(LookupError) ?)

just so that catches can be more explicit and not catch other generic
Key/Lookup Errors. Thoughts? (Think of a better name for it? I
definitely think it should inherit from LookupError, and not just
Exception..)

- Luke