FS.mkdir() will throw a javascript exception if the scratch directory already
exists, so catch/ignore that.
Wrap the rest of the scratch i/o in a try/catch block; the event will only
send if everything works out.
Wrap some calls from Javascript to the C runtime's free() in an
EMSCRIPTEN_KEEPALIVE function, so that the compiler doesn't optimize the
function out and crash at runtime.
Fixes#14999.
267e681a0b