fix #29429, make local-sparam conflict an error instead of warning

[JeffBezanson]

fix #29429

[iblislin]

On the same FreeBSD CI worker:

• ❌ https://build.julialang.org/#/builders/68/builds/1098
• ✔️ https://build.julialang.org/#/builders/68/builds/1101

      From worker 5:	Assertion failed: (e && ptls->bt_size != 0), function jl_sig_throw, file /usr/home/julia/julia-fbsd-buildbot/worker/freebsdci/build/src/task.c, line 437.
      From worker 5:	
      From worker 5:	signal (6): Abort trap
      From worker 5:	in expression starting at /usr/home/julia/julia-fbsd-buildbot/worker/freebsdci/build/test/subtype.jl:1185
      From worker 5:	_thr_kill at /lib/libc.so.7 (unknown line)
      From worker 5:	__raise at /lib/libc.so.7 (unknown line)
      From worker 5:	abort at /lib/libc.so.7 (unknown line)
      From worker 5:	__assert at /lib/libc.so.7 (unknown line)
      From worker 5:	jl_sig_throw at /usr/home/julia/julia-fbsd-buildbot/worker/freebsdci/build/src/task.c:437
      From worker 5:	unknown function (ip: 0xffffffffffffffff)
      From worker 5:	Allocations: 45133200 (Pool: 45109891; Big: 23309); GC: 82