Improve 'either' error messages

[charlespascoe]

Improve either error messages, so that it is clear what each error message relates to.

Idea: A generic interface that has a string (a short description of the type) and the assertion. either takes a number of objects that conforms to this interface. Create a function is that takes a string and an assertion and returns an object that conforms to this interface. Example:

either(
    is('a foo', conformsTo(fooValidator)),
    is('a bar', conformsTo(barValidator))
)