Error reporting

A lot of effort has been put into making error reporting as good as possible in OurBigBook, to allow authors to quickly find what is wrong with their source code.

Error reporting is for example tested with assert_error tests in test.js.

Please report any error reporting bug you find, as it will be seriously tracked under the: error-reporting label.

Notably, OurBigBook should never throw an exception due to a syntax error, as that prevents error messages from being output at all.