[FUSE] Missing error on nonexistent method

[ryzngard]

2 from #8202 (comment)

Missing error for missing callback. OnCopy does not exist

<button class="btn btn-primary" @onclick="IncrementCount" @oncontextmenu="OnCopy">Click me</button>

@code {
    private int currentCount = 0;

    private void IncrementCount()
    {
        currentCount++;
    }
}

[ryzngard]

Figured this out: we're missing source mappings from the compiler here. Screenshot where <# shows source mappings available

Here is what is in main