`[[DEPENDENCY.additional]]`: add diagnostics for missing `license-file`s

**Is your feature request related to a problem? Please describe.**
It appears `cargo-about` will silently fail to generate license text for [`[[DEPPENDENCY.additional]]`](https://github.com/EmbarkStudios/cargo-about#dependencyadditional) tags which only specify `license-file` and not `license`. This might occur if:

* The license file is outside of the crate (e.g. using a relative path like `../../LICENSE-MIT`, **like I was trying to**)
* The license filename or path is typoed (e.g. `LICENSE-MOT`)
* The license file was moved without updating `about.toml` (e.g. from `LICENSE-MIT` to `LICENSE-MIT.md`)

**Describe the solution you'd like**
Some kind of error, warning, or other diagnostic if a `license-file` isn't resolved.

**Describe alternatives you've considered**
My own needs for this feature are mooted by "Add a `--workspace` flag " https://github.com/EmbarkStudios/cargo-about/issues/151 , but I imagine others will accidentally omit licenses trying to use this feature.