Fix interpolation using %s instead of %#v for interfaces

[effeix]

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.