Add more error handling to bp2::ext cwd.ipp. (#354) #351

Job	Run time
windows (msvc-14.0, 14,latest, 32,64, windows-2019)	6h 0m 12s
windows (msvc-14.2, 14,17,20,latest, 32,64, windows-2019)	6h 0m 11s
posix (gcc-5, 11,14,1z, ubuntu-18.04, g++-5)	1s
windows (msvc-14.3, 14,17,20,latest, 32,64, windows-2022)	6h 0m 11s
posix (gcc-6, 11,14,1z, ubuntu-18.04, g++-6)	1s
windows (clang-win, 14,17,latest, 32,64, windows-2022)	6h 0m 10s
posix (gcc-7, 11,14,17, ubuntu-18.04)	1s
windows (gcc, 11,14,17,2a, 64, windows-2019)	1h 19m 29s
posix (gcc-10, 11,14,17,2a, ubuntu-20.04, g++-10)	45m 32s
posix (gcc-12, 11,14,17,20,2b, ubuntu-22.04, g++-12)	1h 5m 45s
posix (clang, clang++-3.9, 11,14, ubuntu-18.04, clang-3.9)	1s
posix (clang, clang++-4.0, 11,14, ubuntu-18.04, clang-4.0)	1s
posix (clang, clang++-5.0, 11,14,1z, ubuntu-18.04, clang-5.0)	1s
posix (clang, clang++-6.0, 11,14,17, ubuntu-18.04, clang-6.0)	1s
posix (clang, clang++-7, 11,14,17, ubuntu-18.04, clang-7)	1s
posix (clang, clang++-8, 11,14,17, ubuntu-20.04, clang-8)	29m 7s
posix (clang, clang++-9, 11,14,17,2a, ubuntu-20.04, clang-9)	39m 54s
posix (clang, clang++-10, 11,14,17,2a, ubuntu-20.04, clang-10)	43m 49s
posix (clang, clang++-11, 11,14,17,2a, ubuntu-20.04, clang-11)	42m 45s
posix (clang, clang++-12, 11,14,17,2a, ubuntu-20.04, clang-12)	42m 27s
posix (clang, clang++-13, 11,14,17,20,2b, ubuntu-22.04, clang-13)	44m 15s
posix (clang, clang++-14, 11,14,17,20,2b, ubuntu-22.04, clang-14)	44m 1s
posix (clang, 11,14,17,2a, macos-11)	1h 2m 24s
	1d 9h 0m 20s

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