Use `ConnectError.unavailable` when client is offline #808

Job	Run time
build-library-tvos	1m 35s
build-eliza-swiftpm-example	1m 25s
build-library-ios	1m 25s
build-library-watchos	1m 31s
build-plugin-and-generate	1m 7s
run-conformance-tests	2m 23s
run-swiftlint	9s
run-unit-tests	2m 30s
validate-license-headers	16s
build-eliza-cocoapods-example	57s
build-library-macos	1m 25s
	14m 43s

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