Add signal handler and early exit/snapshot to Solver.

Add signal handler and early exit/snapshot to Solver.

Add signal handler and early exit/snapshot to Solver.

Add signal handler and early exit/snapshot to Solver.

Also check for exit and snapshot when testing.

Skip running test after early exit.

Fix more lint.

Rebase on master.

Finish rebase on master.

Fixups per review comments.

Redress review comments.

Lint.

Correct error message wording.