R1:000760b645b7

Do a better job of handling spec errors during schema adjustment

Summary:
Ref T1191. Currently if a developer forgot to specify a column type, `storage adjust` aborts explosively mid-stream. Instead:

- Make this a formal error with an unambiugous name/description instead of something you sort of infer by seeing "<unknown>".
- Make this error prevent generation of adjustment warnings, so we don't try to `ALTER TABLE t CHANGE COLUMN c <unknown>`, which is nonsense.
- When schemata errors exist…