Throw slightly better error message when a context variable is not provided

[charlespwd]

Instead of throwing

'A value must be provided for: ' + id, 

I throw

`The intl string context variable '${e.variableId}' was not provided to the string '${message}`

It makes for easier debugging in production.

[yahoocla]

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[jasonmit]

@charlespwd can you add tests?

[charlespwd]

Sure, how about that?

[jasonmit]

Thanks!

[jasonmit]

@okuryu please cut a 2.1.0 release when you can.

[okuryu]

@jasonmit sure, I'll work on it within a week! thanks for your helping. 😎

[Floriferous]

Can you actually use other in a select message? It seems like the code is setup for it, but this error will throw regardless of an other existing in the message.

For example {gender, select, M {Male} F {Female} other {Someone}} will throw if no gender variable has been provided, is that normal?