Change error mock to be more lenient

[Potherca]

This MR makes it possible for the Error Mock middleware to accept nested path and paths without trailing slash.

Currently, only paths exactly matching /xxx/ are caught. This MR adds support for /xxx, /n/xxx and /n/xxx/.

Obviously, tests have been added to verify this new behaviour.

[poef]

What is the use case for /nested/500/ matching? (Or any other error code after any initial path)

[Potherca]

The use-case is:

• When a developer wants to use a different domain for consistency with other test/debug calls¹, such a domain might contain a non-root URL for status codes²

Footnotes

1. For instance https://httpbin.org/ ↩

2. For instance https://httpbin.org/status/500 ↩

[Potherca]

Discussed offline: the "trigger" should be unique-ish, this would also allow it to be nested. To make it uniquely idnetifiable /STATUS_xxx should be used, for instance https://example.com/whatever/STATUS_4000.