Unions of instances produce unhelpful error message

[matthew-dean]

Problem

If a struct contains an instance type

const Struct = type({
  condition: instance(Condition)
})
//...

...you get a helpful error message like:

Expected a `Condition` instance, but...

However, if you use a union:

const Struct = type({
  condition: union([instance(Condition), instance(Bool)])
})

...The error message ceases to have any usefulness, and outputs:

Expected the value to satisfy a union of `instance | instance`...

Possible Solutions

1. Output a message like:

   Expected the value to satisfy a union of `Condition | Bool`...

2. Instead of leaving it up to Superstruct, allow customization of messages in the struct, like:

const Struct = type({
  condition: label(union([instance(Condition), instance(Bool)]), 'Condition | Bool')
})

...which could produce:

Expected the value to satisfy `Condition | Bool`, but...