refactor(tokio_util::codec): Return unparsed bytes on failure.

[coveralls]

Pull Request Test Coverage Report for Build 5520035435

• 29 of 29 (100.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.007%) to 93.247%

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Files with Coverage Reduction	New Missed Lines	%
imap-codec/src/tokio/client.rs	1	91.36%
imap-codec/src/tokio/server.rs	1	88.89%
<!--	Total:	2		-->

Totals
Change from base Build 5465650402:	-0.007%
Covered Lines:	9141
Relevant Lines:	9803

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💛 - Coveralls