Better handling of invalid WSGI path encoding

Split from #1685: as reported by @rokclimb15, in some cases the WSGI path may Unicode characters that don't comply with the WSGI spec. According to [PEP 3333](https://peps.python.org/pep-3333/#unicode-issues):
> On Python platforms where the `str` or `StringType` type is in fact Unicode-based (e.g. Jython, IronPython, Python 3, etc.), all “strings” referred to in this specification must contain only code points representable in ISO-8859-1 encoding (`\u0000` through `\u00FF`, inclusive). It is a fatal error for an application to supply strings containing any other Unicode character or code point. Similarly, servers and gateways **must not** supply strings to an application containing any other Unicode characters.

Per this definition, Falcon actually handles this correctly by exploding with an unhandled error, that is what a fatal error is.

However, if it is not too expensive to catch this error, maybe we could render an HTTP 400 response anyway, providing a helpful message explaining what was the actual problem? Or alternatively, bubble up an unhandled error, but provide a more helpful message explaining what exactly is going on (with a reference to the spec).

Start by tracing Falcon's WSGI path handling and the unhandled error produced for non-ISO-8859-1 characters. Read PEP 3333's Unicode issues guidance alongside the existing behavior. Done means deciding whether to return HTTP 400 with a helpful message or preserve the unhandled error while making its cause and specification reference clear.

Written by the indexing model from the issue text.