feat(test2): Allow use of custom error types

[epage]

This is part of #13

[coveralls]

Pull Request Test Coverage Report for Build 18919021191

Details

• 0 of 23 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 30.588%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/libtest2-mimic/src/lib.rs	0	2	0.0%
crates/libtest2-harness/src/error.rs	0	21	0.0%

Totals
Change from base Build 18856174810:	-0.1%
Covered Lines:	505
Relevant Lines:	1651

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💛 - Coveralls