Migrate value-returning error sites to metacall_error_throw

[ROKUMATE]

Summary

Replaces log_write + return NULL with metacall_error_throw across
value-returning entrypoints in metacall.c and the loaders, so errors
propagate as throwable values instead of being silently swallowed.

Pattern follows rapid_json_serial_impl.cpp:422:

  return metacall_error_throw("metacall", -1, NULL, "<message>", <args>);

Scope

In:

• value-returning functions (return type void * / value)
• removes the old log + NULL return; single throw statement

Out:

• int-returning functions
• metacall_error_last / metacall_error_clear (still TODO)
• stacktrace propagation
• debug-mode log inside metacall_error_throw

Progress

• handle-validation entrypoints (metacallhv, metacallhv_s, metacallht_s, metacall_handle_function, metacall_handle_export)
• "method not implemented" cluster (metacallv_method and friends)
• remaining value-returning sites in metacall.c
• per-loader passes (py_loader, node_loader, ...)