Add test stub that always fail to escape or quote

[ericcornelissen]

Relates to #1149

Summary

Expand shescape/testing with a stub implementation that never fails to instantiate but always fails when used to .escape(All) or .quote(All) an argument.

The use case of this stub is to simulate what the target code does when shescape can't escape or quote the provided argument. As such, it would sit in between Stubscape and Throwscape as:

• Stubscape: Nearly identical behavior to Shescape, but never fails to instantiate.
• Failscape: Always fails to escape or quote, but never fails to instantiate.
• Throwscape: Always fails to instantiate.

The key difference between Stubscape and Failscape is that the stub user won't have to worry about getting an invalid argument to the shescape call site, rather it just fails if that what needs to be tested.