When a handler cannot be found for a given path, previously Fiber
would construct a plaintext response that cannot be modified.

This commit switches to returning a new instance of `*fiber.Error`
with identical error message so that users can customise the look
of their 404 pages.

Signed-off-by: AKP <tom@tdpain.net>

This test was failing as the error returned by `c.Next()` that's
required to generate the correct 404 status code was not being
passed through the middleware and being silently ignored.

Signed-off-by: AKP <tom@tdpain.net>

Signed-off-by: AKP <tom@tdpain.net>

As far as I can tell, this test is meant to check that a cached
HEAD request to a given endpoint does not return the cached
content to a GET request to the same endpoint, and the test has
been altered to correctly check for this.

Signed-off-by: AKP <tom@tdpain.net>