refactor: Simplify `Error`s.

[coveralls]

Pull Request Test Coverage Report for Build 5808595175

• 183 of 201 (91.04%) changed or added relevant lines in 10 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.05%) to 93.444%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
imap-types/src/datetime.rs	1	2	50.0%
imap-types/src/extensions/move.rs	0	1	0.0%
imap-types/src/mailbox.rs	7	8	87.5%
imap-types/src/extensions/quota.rs	0	3	0.0%
imap-types/src/response.rs	3	6	50.0%
imap-types/src/command.rs	1	5	20.0%
imap-types/src/sequence.rs	10	15	66.67%
<!--	Total:	183	201	91.04%	-->

Files with Coverage Reduction	New Missed Lines	%
imap-types/src/sequence.rs	1	90.91%
<!--	Total:	1		-->

Totals
Change from base Build 5799440177:	0.05%
Covered Lines:	8794
Relevant Lines:	9411

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💛 - Coveralls