Fix get-custom-snapshots to conform to the docs.
Check for incomplete blame information and error.
Expose missing blame parties to the contract profiler.

Done by pairing up incomplete blame objects with the missing blame party
before attaching it to the continuation mark. Other approaches were
explored (having a separate mark for each, imperatively updating blame
objects to add missing blame parties, etc.), but pairing had the least
overhead.
Fix `for` to propagate source locations in more cases.

Thanks to Tobias Hammer for the patch.

Closes PR 14343.
Fix TR optimizer tests for new source locations.
Fix doc contract for list-box%
Remove predicate checks superceded by contracts
Contracts for racket/gui.
Improve timer% contract
Update outdated and/or wrong GUI contracts