urlTemplate does not complain about missing inputs

[dglazkov]

Unlike promptTemplate, the urlTemplate happily substitutes blanks if the inputs aren't supplied. Instead, it should complain about missing inputs like promptTemplate.

Discovered by @jig925 in https://github.com/breadboard-ai/breadboard/discussions/1182#discussioncomment-9094597

[PaulKinlan]

It would also be worthwhile documenting some of the syntax for urlTemplate too, it's uses a very robust library that can do cool things like build query strings correctly.