Provide hook to prevent alerts from being shown when logging
Make browser assertions throw a proper Error type instead of a string

Error objects can have stack traces and other things to make diagnosing
asserts easier.
Remove unused 3rd parameter of runtime.assert

This 3rd callback parameter is never used inside the current WebODF
source.
Standardize all runtime assert implementations
Merge pull request #731 from peitschie/disable-alert

Provide hook to prevent browser alerts from being shown when logging