Blockly crashes completly when the local browser storage is invalid.

[BjoernLuig]

Current behaviour

Blockly crashes completly when the local browser storage is invalid. For example a sketch with a block that got renamed or is no longer supported. This happened while developing.

Should this ever happen for the end user, because of an unexpected error on our site (no longer supporting an old block, ...) he would only see a blank white website without any error message or suggestion to remove the browser cookies. Blockly would seem to be completly unusable. See screenshot below where the block was renamed to "wrong block" for demonstration:

Expected behaviour

If the local browser storage is invalid, it should be reset uncluding a short notification.