Report a backend error naming the builtin instead of raising NotImplementedError on wasm

[zimri-leisher]

rand(), randf() and set_seed() have no llvm/wasm lowering, and the placeholder generate_llvm raised NotImplementedError — which is neither a CompileError nor a BackendError, so compile_main didn't catch it and the user got a traceback:

$ fprime-fpyc rnd.fpy -d TopologyDictionary.json --emit wasm
...
NotImplementedError: this builtin has no LLVM/wasm lowering yet

generate_llvm now defaults to None and the backend raises a BackendError at the call, so it points at the source like every other backend error:

rnd.fpy:1 the 'rand' builtin is not supported by the wasm backend
     1 | x: U32 = rand()
                  ^^^^^^

It is still a hard error — nothing is silently miscompiled.