pxf: DatasetReader.scan forces skipValidate, keeps MaxMessageSize, and positions every row error #194

protowire-go is changing DatasetReader.Scan (trendvidia/protowire-go#280, PR trendvidia/protowire-go#281; decided in trendvidia/protowire-cpp#166). Scan will bind each row under all the caller's options except MaxMessageSize, as the options-taking BindRow does. When a row is bound, only a decode error gets the row's position (2:1: column pw: ...). A schema violation (PXF schema violations: ...) and a validator's error (check: ...) come back as they are, with no position. protowire-cpp already behaves this way (PR trendvidia/protowire-cpp#174).

What this port does (from reading the code at c685ff0, not run)

• DatasetReader.scan passes the reader's options, but BindRow.bind sets withSkipValidate(true). The comment says the descriptor was already validated, but no such check was found in DatasetReader. If so, a schema violation can never come out of scan, while Go reports one.
• MaxMessageSize is not lifted for the row's body. Go binds at the default, because the body repeats the column names and is wider than the row.
• atRow (BindRow.java L206–216) gives every caught PxfException the row's position, schema violations included.
• The port has no secret-hook or validator option, so those cases don't apply.

Done when

• Each option the port has goes through Scan, with a test per option. The cases to copy are in protowire-go's encoding/pxf/dataset_scan_options_test.go on #281: the same row bound through Scan and through BindRow must give the same message or the same error, and the texts must match.
• Row errors are positioned as Go positions them.

Blocked by trendvidia/protowire-go#280 (merge of #281).