P3417 R1 Handling exceptions thrown from contract predicates

[wg21bot]

P3417R0 Improving the handling of exceptions thrown from contract predicates (Gašper Ažman, Timur Doumler)

[wg21bot]

P3417R1 Handling exceptions thrown from contract predicates (Gašper Ažman, Timur Doumler)

[timuraudio]

R1 no longer proposes anything; closing.