fix(presets): add --failure-level ERROR to asciidoctor args

Problem: asciidoctor exits 0 even when it encounters ERROR-level
messages, so the compiler treats every run as a success and never
invokes error_parser. Errors are silently embedded as visible text
in the HTML output instead of surfacing as diagnostics.

Solution: pass --failure-level ERROR so asciidoctor exits non-zero
on errors, which triggers error_parser and surfaces diagnostics.

spec/presets_spec.lua

@@ -467,7 +467,7 @@ describe('presets', function()
 
     it('returns args with file and output', function()
       assert.are.same(
-        { '/tmp/document.adoc', '-o', '/tmp/document.html' },
+        { '--failure-level', 'ERROR', '/tmp/document.adoc', '-o', '/tmp/document.html' },
         presets.asciidoctor.args(adoc_ctx)
       )
     end)