fix error message on equality check

[chenyan-dfinity]

Fix #454

[github-actions]

Benchmark for 0d222a6

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Test	Base	PR	%
Blob/&str	229.3±111.70µs	229.0±111.17µs	-0.13%
Blob/ByteBuf	151.6±47.64µs	150.8±47.98µs	-0.53%
Blob/Bytes	108.2±43.72µs	107.1±43.58µs	-1.02%
Blob/String	302.1±148.61µs	266.7±125.75µs	-11.72%
Collections/vec (text, nat)	68.0±0.85ms	69.5±1.09ms	+2.21%
Collections/vec int	35.4±0.07ms	32.6±0.16ms	-7.91%
Collections/vec int64	18.9±0.23ms	18.6±0.33ms	-1.59%
Collections/vec nat8	13.7±0.07ms	13.8±0.02ms	+0.73%
option list/1024	1384.4±1.03µs	1382.1±2.79µs	-0.17%
profiles/1024	2.7±0.01ms	2.6±0.01ms	-3.70%
variant list/1024	1134.2±2.46µs	1146.3±2.60µs	+1.07%

[github-actions]

Benchmark for 98acdf8

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Test	Base	PR	%
Blob/&str	294.0±89.46µs	294.4±88.09µs	+0.14%
Blob/ByteBuf	244.6±64.97µs	236.0±72.68µs	-3.52%
Blob/Bytes	168.4±48.95µs	163.1±47.14µs	-3.15%
Blob/String	348.8±87.38µs	380.1±108.73µs	+8.97%
Collections/vec (text, nat)	92.9±4.66ms	96.4±4.58ms	+3.77%
Collections/vec int	42.6±1.88ms	42.6±2.15ms	0.00%
Collections/vec int64	24.0±1.24ms	24.7±1.42ms	+2.92%
Collections/vec nat8	17.5±0.66ms	18.8±1.38ms	+7.43%
option list/1024	1823.0±86.78µs	1847.6±100.51µs	+1.35%
profiles/1024	3.5±0.17ms	3.6±0.31ms	+2.86%
variant list/1024	1456.5±70.21µs	1525.3±143.89µs	+4.72%