Attempting to render a template with missing variables should raise an error

[Robsteranium]

It's possible to render a uri template with a variable that doesn't exist.

The result is that an empty string (nothing) is interpolated into the template.

This is probably never intentional - perhaps having a variable explicitly defined as "" but certainly never an implicit empty string.

We ought to raise an exception e.g.

template variable foo is undefined in template http://example.net/{foo}, available variables: bar, baz