Make template resolving failure an error

[Flamefire]

Currently resolve_template simply shows a warning when it cannot resolve a template.

I'm proposing to change that to a hard error. This is made possible after #3129 was merged which resolves the internal resolving failures.

Reasoning: Usually it is wanted that all template values are resolved. For cases where it isn't wanted there is the %%(foo)s syntax (or any other even number of % signs). Failure to resolve a template value will lead to failures later down potentially after wasting a lot of time doing other work until e.g. a sanity-check command fails. Example: #3218

Throwing an error allows to pinpoint the exact location where a value is resolved leading to faster bugfixes and is also better for users to show a mistake on their side as early as possible.

[Flamefire]

Implemented by now