Exit migration if existing rocksdb ledger found (#4778) #2417

Job	Run time
Linux [UBSAN] [lmdb \| clang]	40m 37s
macOS [UBSAN] [lmdb \| clang]	32m 41s
Linux [ASAN] [lmdb \| clang]	39m 43s
macOS [ASAN] [lmdb \| clang]	28m 12s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	40m 55s
macOS [TSAN] [lmdb \| clang]	32m 19s
Linux [TSAN] [lmdb \| clang]	40m 48s
Linux [LEAK] [lmdb \| clang]	38m 9s
macOS [UBSAN] [rocksdb \| clang]	42m 10s
Linux [UBSAN] [rocksdb \| clang]	49m 32s
macOS [ASAN] [rocksdb \| clang]	38m 35s
Linux [ASAN] [rocksdb \| clang]	51m 36s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	54m 2s
macOS [TSAN] [rocksdb \| clang]	50m 56s
Linux [LEAK] [rocksdb \| clang]	52m 44s
	10h 32m 59s

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