js> load('testsrc/doctests/util.js')
js> var str = Error.prototype.toString
js> str.call(new TypeError("msg"))
TypeError: msg
js> str.call(new TypeError()) // message is initialised to ''
TypeError: 
js> str.call(new Error("msg"))
Error: msg
js> str.call(new Error()) // message is initialised to ''
Error: 
js> str.call({name:"my error", message:"my message"})
my error: my message
js> str.call({}) === undefined
true
js> str.call({name:"no message defined"}) === undefined
true
js> str.call({name:"message is undefined", message:undefined}) === undefined
true
js> str.call({name:"null message", message:null})
null message: null
js> str.call({message:"no name defined"})
Error: no name defined
js> str.call({name:undefined, message:"name is undefined"})
Error: name is undefined
js> str.call({name:null, message:"null name"})
null: null name