collation-generation: guide and tidying (#1753)

* Guide: Change for {Global,Local}ValidationData to ValidationData

Seems like that was missed during the transition from notions of global and local validation data to persistent and transient

* collation-generation: a small drive-by fix of line length

* collation-generation: extract an error module

this allows us to avoid piling Error and Result types from different contexts, specifically std vs. module-local

* collation-generation: extract LOG_TARGET from log statements