Async compilation improvements

[kripken]

• When async compilation fails, we try sync compilation as a fallback, but the error message just mentioned sync. Update it to mention both, as likely the user would want to fix async compilation, and not care about the tricks needed for sync compilation.
• Update the tutorial text: now that we emit wasm by default, emitting an html file requires loading of a wasm in an xhr, and so a webserver is needed for browsers that don't support file:// loading, which it seems is all browsers but firefox these days.