UI: Build wasm with -g2 to preserve symbols, fix OOM message

The previous emscripten+wasm toolchain roll broke C++ symbols
in Wasm crashes and, recursively, the special casing around
known crashes (e.g. OOM).
This CL:
- Adds -g2 to preserve function names, now explicitly required
  when building with -O3.
- Adds ASSERTIONS=1 (1 in debug builds) to get the error message
  ("cannot enlarge memory") from OOMs.
- Tweaks the OOM detection code, as the message slightly changed
  (it doesn't mention "array" anymore).
- Makes the OOM message a bit more actionable.

Bug: 182711686
Bug: 181218057
Bug: 181237261
Change-Id: I84909f08bd2d93b4545cd5f0a5e29f94dd126bee