[[DEPENDENCY.additional]]: add diagnostics for missing license-files

[MaulingMonkey]

Is your feature request related to a problem? Please describe.
It appears cargo-about will silently fail to generate license text for [[DEPPENDENCY.additional]] 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 " #151 , but I imagine others will accidentally omit licenses trying to use this feature.