unlink() does not check errors and empty results

[cbratschi]

The following code in schema.js should be changed from:

GridFSSchema.statics.unlinkById =
        GridFSSchema.statics.unlink = function(id, done) {
            this.findById(id).exec(function(error, foundInstance) {
                foundInstance.unlink(done);
            });
        };

To:

GridFSSchema.statics.unlinkById =
        GridFSSchema.statics.unlink = function(id, done) {
            this.findById(id).exec((error, foundInstance) => {
                if (error) {
                     return done(error);
                }

                if (!foundInstance) {
                    return done(new Error('not found'));
                }

                foundInstance.unlink(done);
            });
        };

Currently if the file does not exist in GridFS the whole app will crash.

[lykmapipo]

@cbratschi I will appreciate PR for this.

Thanks.

[lykmapipo]

@cbratschi Fixed and PR merged. Thanks