Remove unwrap from Resolver

[zbraniecki]

Resolver code uses some unwrap which should be replaced with error handling.

[zbraniecki]

I cleaned up examples to use expect rather than unwrap which I think is way more meaningful.

What's left are:

• unwraps on FluentValue::as_number
• unwrap inside of a resolver for CallExpression in arg.value.to_value(env).unwrap()

@stasm - thoughts on how we should handle them?

[stasm]

Thanks for summarizing what remains to be done here. I opened #86 with proposed fixes.