assert: fix interpolation using %s instead of %#v for interfaces

[effeix]

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Summary

Fix string interpolation issues using %s instead of %#v

Changes

Switch %s with %#v inside error messages in func Len() use err.Error() instead of only err to log the error message in func NotEqual() Switch %s with %#v inside error messages in func NotContains() Switch %s with %#v inside error messages in func Subset() Switch %s with %#v inside error messages in func NotSubset()

Motivation

The error messages were not clear on what was happening because the element is not being formatted correctly.

[dolmen]

@effeix Please rebase and fix conflicts.