unhandled kwargs throw errors

[ericphanson]

closes https://github.com/JuliaLang/JuliaSyntax.jl/issues/416

My editor is configured to remove trailing whitespace, which caused some diffs here. Let me know if that's too annoying and I'll undo it.

Locally I get the test failures:

Parse core.jl: Test Failed at /Users/eph/JuliaSyntax.jl/test/test_utils.jl:237
  Expression: parsers_agree

Parse core.jl: Test Failed at /Users/eph/JuliaSyntax.jl/test/test_utils.jl:241
  Expression: reduced_failures == []
   Evaluated: AbstractString["for f() in 1:3\n        push!(a, f())\n        push!(fs, f)\n    end", " {{}}where{}"] == Any[]

same as in #420, which I think are unrelated.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.17%. Comparing base (a6f2d15) to head (c63e9c7). Report is 47 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #422 +/- ## ========================================== + Coverage 94.61% 96.17% +1.56% ========================================== Files 14 14 Lines 4178 4185 +7 ========================================== + Hits 3953 4025 +72 + Misses 225 160 -65 ```

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