Ensure checked integers are in a sane state after a throw.

[jzmaddock]

Document that the value of the target operand is unspecified after a throw. Improve testing. Fixes https://github.com/boostorg/multiprecision/issues/626

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.1%. Comparing base (58b3962) to head (5926513).

Additional details and impacted files

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