#50 Skipping a document should show a clear warning if there are reference parsing errors

[Errors processing data files]
 ╿ [Errors during data step: Process artists file]
 │  ╿ [Errors processing documents]
 │  ╎  ╿ (#488) (name: 'koba') Unknown fields present: Alias
The above errors were detected while processing data files.
If the remaining valid data is complete enough, the wiki will
still build - but all errored data will be skipped.
(Resolve errors for more complete output!)
No duplicate directories found - nice!
[Errors validating between-thing references in data]
 ╿ [Reference errors in wikiData.trackData]
 │  ╿ [Reference errors in Track "licord nacrasty" (track:licord-nacrasty) (#8 in cool and new voulem.1)]
 │  ╎  ╿ [Reference errors in contributions artistContribsByRef (find.artist)]
 │  ╎  │  ╿ (#1) Didn't match anything for artist:kobacat
 │  ╎ [Reference errors in Track "upp word(s) movmeant (Fanon Edit)" (track:upp-words-movmeant-fanon-edit) (#26 in cool and new voulem.1)]
 │  ╎  ╿ [Reference errors in contributions artistContribsByRef (find.artist)]
 │  ╎  │  ╿ (#1) Didn't match anything for artist:kobacat
 │  ╎ [Reference errors in Track "PAST FUNK IS WORSE THAN FUTURE FUNK" (track:past-funk-is-worse-than-future-funk) (#48 in Land of Fans and Music 5)]
 │  ╎  ╿ [Reference errors in contributions artistContribsByRef (find.artist)]
 │  ╎  │  ╿ (#1) Didn't match anything for artist:kobacat
The above errors were detected while validating references in data files.
If the remaining valid data is complete enough, the wiki will still build -
but all errored references will be skipped.
(Resolve errors for more complete output!)

We already show a specific message if duplicate directories were found (resulting in documents being exempted from processing), but skipping due to errors in processing YAML documents should also be explicitly indicated. See confusion with the following output: ``` [Errors processing data files] ╿ [Errors during data step: Process artists file] │ ╿ [Errors processing documents] │ ╎ ╿ (#488) (name: 'koba') Unknown fields present: Alias The above errors were detected while processing data files. If the remaining valid data is complete enough, the wiki will still build - but all errored data will be skipped. (Resolve errors for more complete output!) No duplicate directories found - nice! [Errors validating between-thing references in data] ╿ [Reference errors in wikiData.trackData] │ ╿ [Reference errors in Track "licord nacrasty" (track:licord-nacrasty) (#8 in cool and new voulem.1)] │ ╎ ╿ [Reference errors in contributions artistContribsByRef (find.artist)] │ ╎ │ ╿ (#1) Didn't match anything for artist:kobacat │ ╎ [Reference errors in Track "upp word(s) movmeant (Fanon Edit)" (track:upp-words-movmeant-fanon-edit) (#26 in cool and new voulem.1)] │ ╎ ╿ [Reference errors in contributions artistContribsByRef (find.artist)] │ ╎ │ ╿ (#1) Didn't match anything for artist:kobacat │ ╎ [Reference errors in Track "PAST FUNK IS WORSE THAN FUTURE FUNK" (track:past-funk-is-worse-than-future-funk) (#48 in Land of Fans and Music 5)] │ ╎ ╿ [Reference errors in contributions artistContribsByRef (find.artist)] │ ╎ │ ╿ (#1) Didn't match anything for artist:kobacat The above errors were detected while validating references in data files. If the remaining valid data is complete enough, the wiki will still build - but all errored references will be skipped. (Resolve errors for more complete output!) ``` It was unclear that the unknown field "Alias" (rather than "Aliases") being present was causing the artist document to be skipped altogether, resulting in an error message. It could also be nice to show a summary of what *kinds* of documents were skipped (by both duplicated directories and YAML processing errors), such as "Some documents weren't included due to processing errors: 2 Artists, 3 Tracks", just to make it even more explicit that there is Something Specific Going Wrong, but this isn't outright needed (would just be nice).