[wasm] build with -fexceptions

[nornagon]

Summary

None

Purpose of change

The emscripten build has been crashing whenever an exception is thrown, which
turns out to be actually surprisingly rarely! But #71760 started making
something throw during JSON loading. In normal builds it's caught and handled
transparently, but without -fexceptions, emscripten will translate C++
exceptions into a full halt of the program, just as if the exception wasn't
caught.

Describe the solution

Enabling -fexceptions does have a performance impact, but being unable to play
the game seems worse than being able to play it but a bit more slowly, so here
we go :D

Describe alternatives you've considered

The particular exception that was being caught was during JSON deserialization,
which seems possible to rewrite not to use exceptions. But exceptions seem
fairly widely used in the project, so let's just enable them for now and see
how far that gets us. Eventually emscripten will support asyncify +
-fwasm-exceptions for wasm-native exception handling and the perf impact will
go away.

Testing

Game loads under wasm now!

Additional context