I'm not sure how much of wmerrors need to be ported. HHVM appears to enhance PHP's error reporting capabilities in various (alas, mostly undocumented) ways, so perhaps some of the fancy footwork in wmerrors won't need to be ported. But at minimum, php-fatal-error.html should work.
See onFatalError, onUnhandledException in hphp/runtime/base/execution-context.cpp
Version: wmf-deployment
Severity: enhancement