Make the asmjs version run even when there's no SIMD available

Instead of throwing an error, it's better to just inform the user that SIMD is not enabled, the non-SIMD version is still available.

I created a nonSimdAsmjsModule with all the SIMD stuff stripped out.  Probably not the most elegant way to fix it.

Also, there was an alert() in the worker-asm version.  Alerts cannot be issued from web-workers.