* Recover from invalid `'label: ` before block.
* Make suggestion to enclose statements in a block multipart.
* Point at `match`, `while`, `loop` and `unsafe` keywords when failing
to parse their expression.
* Do not suggest `{ ; }`.
* Do not suggest `|` when very unlikely to be what was wanted (in `let`
statements).
Impressing confused Python users with magical diagnostics is perhaps
worth this not-grossly-unreasonable (only 40ish lines) extra complexity
in the parser?
Thanks to Vadim Petrochenkov for guidance.
This resolves#46836.
