fix: compilation.errors should contain Error objects

[henryqdineen]

We ran into an issue where HtmlWebpackPlugin where Rspack would crash with an error

/node_modules/@rspack/core/dist/index.js:1370
            push: (...newItems)=>(adm.spliceWithArray(adm.length, 0, newItems), adm.length),
                                      ^

Error: Missing field name
    at Proxy.push (/node_modules/@rspack/core/dist/index.js:1370:39)

I tracked it down to some rust code in Rspack. My fix in this PR is to just wrap the pretty-error string in an Error which would also be more accurate based on both the webpack/Rspack types.

An alternative I considered was just not adding the raw error to compilation.errors. Would that be preferred?

Thanks!

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