error handling if browser does not allow access to databases

[mojoaxel]

It is possible to disable "Allow websites caches and databases" e.g. in IE10/11.

If databases are disabled this.idb is null. This should be handled in the openDB function.

[jensarps]

Agreed, a more meaningful error message, and the option to handle this is a good thing. Awesome that you did the implementation already – many thanks!