catch `!` in yaml parse error messages

[cscheid]

We should offer a better error message than this: #2763 (reply in thread)

[cscheid]

The new error message is (slightly) more informative:

$ quarto render
Check file:///Users/cscheid/repos/github/quarto-dev/quarto-cli/src/quarto.ts
ERROR: YAMLError: unknown tag !<!post/index.qmd> at line 8, column 1:

    ^
Did you try to use a '!' in a YAML string? If so, you need to add explicit quotes to your string.

As is obvious, lots more work is still necessary. But this fix stops this particular booboo.