output envconfig name if missing instead of generic extra resource name

[stefnans]

Display the envconfig name in case it's missing

Executing

$ crossplane render --extra-resources example/environmentConfigs.yaml --include-context example/xr.yaml example/composition.yaml example/functions.yaml

Will output the name of the missing envconfig in the error message:

crossplane: error: cannot render composite resource: pipeline step "environmentConfigs" returned a fatal result: cannot get selected environment configs: Required environment config "missing-envconfig" not found

instead of

crossplane: error: cannot render composite resource: pipeline step "environmentConfigs" returned a fatal result: cannot get selected environment configs: Required environment config "environment-config-0" not found

Fixes #60

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.

[phisco]

Thanks, @stefnans! Sounds reasonable!