Add error/validation handler with default display interface

[tnightingale]

Need a nice way of exposing issues/errors to users in a friendly way.

Eg:

• their GeoJSON is invalid
• they're trying to add more values than the cardinality allows

Should be easily overridable by developers to allow for integration into existing error messaging systems.

Pay attention to this: https://github.com/jacobtoye/Leaflet.draw/issues/33 as a 'disabled' button state would also be useful for the cardinality thing.