Throw error for missing URL template values
Throw error for missing URL template values

We want it to be immediately obvious when values we expect to use in a
URL template are missing or null. This involes refactoring the
templating code, otherwise values from the context would just never get
replaced.

ebmdatalab/openprescribing,annapowellsmith/openpresc,annapowellsmith/openpresc,ebmdatalab/openprescribing,annapowellsmith/openpresc,annapowellsmith/openpresc,ebmdatalab/openprescribing,ebmdatalab/openprescribing