Add `err2str` mirroring `av_err2str` #371

Job	Run time
build_dynamic_and_test_windows_pre	5m 59s
docs_rs_check	57s
rust_clippy_check_ubuntu	5m 9s
build_static_and_test_ubuntu (release/6.0, nightly, no valgrind)	5m 45s
rust_clippy_check_windows	1m 54s
build_static_and_test_ubuntu (release/6.0, 1.70.0, no valgrind)	5m 19s
rustfmt_check (macos-latest)	23s
build_static_and_test_ubuntu (release/6.1, nightly, valgrind)	1h 3m 29s
rustfmt_check (windows-latest)	41s
build_static_and_test_ubuntu (release/6.1, nightly, no valgrind)	6m 36s
rustfmt_check (ubuntu-latest)	17s
build_static_and_test_ubuntu (release/6.1, 1.70.0, no valgrind)	5m 28s
build_dynamic_and_test_windows (nightly)	2m 12s
build_dynamic_and_test_windows (1.70.0)	2m 7s
	1h 46m 16s

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