Check whether files could be opened before writing; resolves #542 #378

Job	Run time
windows-msvc-shared (JSON)	10m 21s
windows-msvc-shared (AVRO)	7m 48s
windows-msvc-shared (CAPNPROTO)	7m 18s
windows-msvc-shared (FLEXBUFFERS)	5m 30s
windows-msvc-shared (PARQUET)	38m 43s
windows-msvc-shared (CBOR)	8m 34s
windows-msvc-shared (MSGPACK)	4m 0s
windows-msvc-shared (YAML)	4m 18s
windows-msvc-shared (UBJSON)	7m 13s
windows-msvc-shared (TOML)	5m 8s
windows-msvc-shared (XML)	3m 49s
windows-msvc (TOML)	4m 11s
windows-msvc (JSON)	10m 34s
windows-msvc (UBJSON)	6m 59s
windows-msvc (XML)	3m 56s
windows-msvc (CBOR)	8m 20s
windows-msvc (AVRO)	7m 25s
windows-msvc (MSGPACK)	3m 47s
windows-msvc (benchmarks)	47m 18s
windows-msvc (CAPNPROTO)	8m 20s
windows-msvc (FLEXBUFFERS)	5m 25s
windows-msvc (YAML)	4m 6s
windows-msvc (PARQUET)	36m 10s
	4h 9m 13s

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