Add error tests

[Nef10]

Refs #1

[github-actions]

Coverage after merging tests into main

20.89%

Coverage Report

File	Stmts	Funcs	Lines	Uncovered Lines
Sources/SwiftBeanCountWealthsimpleMapper
LedgerLookup.swift	57.55%	57.69%	57.52%	115–127, 67–73, 75–84, 86–92
Wealthsimple.Transaction.swift	0%	0%	0%	13–30, 32–34, 36–38
WealthsimpleConversionError.swift	100%	100%	100%
WealthsimpleLedgerMapper.swift	0%	0%	0%	100–129, 143–178, 187–199, 202–235, 237–244, 246–251, 253–268, 270–276, 278–290, 292–298, 300–305, 307–309, 31, 310–314, 317–319, 32, 320–325, 327–329, 33, 330–333, 34–37, 40–43, 46–50, 52–56, 61, 67–69, 71–81, 95–99