wasm32: Unify error codes an implementations

Right now, while our implementations for stdweb and wasm-bindgen use
similar APIs, they do not use similar implementations. This change:

  - Switches to using the same error codes for both implementations
  - Uses error codes that detail exactly what API is missing
  - Cleans up the implemetnations to make them much more readable
  - Consitantly use thread_local in both implementations

This makes issues easier to debug and the code easier to understand.

Signed-off-by: Joe Richey <joerichey@google.com>