Add custom mapping from errors to views

[dannflor]

Adds an optional parameter to init as well as the LeafMiddlewareDefaultGenerator to provide a custom set of mappings of HTTPStatus to .leaf template names. Does not break the public API. I did this because it didn't make sense to get a 503 page when someone messes up their password.

[codecov]

Codecov Report

Merging #23 (16f05a1) into main (2f707d0) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   96.61%   96.82%   +0.21%     
==========================================
  Files           2        2              
  Lines          59       63       +4     
==========================================
+ Hits           57       61       +4     
  Misses          2        2              

Impacted Files	Coverage Δ
...rces/LeafErrorMiddleware/LeafErrorMiddleware.swift	96.00% <100.00%> (+0.08%)	⬆️
...ddleware/LeafErrorMiddlewareDefaultGenerator.swift	100.00% <100.00%> (ø)

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[0xTim]

Whilst we can argue the merits of returning a 401 page instead of using the redirect middleware I think this PR has some useful features in. Some requested changes around the tests then we're good

[dannflor]

@0xTim Bump. Wanna start using these features in my projects 😛

[0xTim]

Thanks!