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

polkadot/roadmap/implementers-guide/src/node/collators/collation-generation.md

@@ -29,7 +29,7 @@ pub struct Collation {
 
 struct CollationGenerationConfig {
   key: CollatorPair,
-  collator: Box<dyn Fn(&GlobalValidationData, &LocalValidationData) -> Box<dyn Future<Output = Collation>>>
+  collator: Box<dyn Fn(&ValidationData) -> Box<dyn Future<Output = Collation>>>
   para_id: ParaId,
 }
 ```