promote_type should fail more gracefully

[jonniedie]

It's pretty easy to mistakenly define promote_type methods when you meant to define promote_rule methods. This can produce a fatal error in some cases, though:

julia> struct MyType{T}
           a::T
       end

julia> Base.promote_type(::Type{<:MyType}, ::Type{T}) where {T<:Number} = MyType{T}

fatal: error thrown and no exception handler available.
MethodError(f=typeof(Base.promote_type)(), args=(Core.TypeofBottom(), Int64), world=0x0000000000006cd8)
jl_method_error_bare at /cygdrive/d/buildbot/worker/package_win64/build/src\gf.c:1798
jl_method_error at /cygdrive/d/buildbot/worker/package_win64/build/src\gf.c:1816
jl_lookup_generic_ at /cygdrive/d/buildbot/worker/package_win64/build/src\gf.c:2373
jl_apply_generic at /cygdrive/d/buildbot/worker/package_win64/build/src\gf.c:2394
promote_result at .\promotion.jl:237 [inlined]
promote_type at .\promotion.jl:223 [inlined]
promote_typeof at .\promotion.jl:262 [inlined]
+ at .\int.jl:919
threadid at .\threadingconstructs.jl:10
task_done_hook at .\task.jl:432
jl_apply at /cygdrive/d/buildbot/worker/package_win64/build/src\julia.h:1690 [inlined]
jl_finish_task at /cygdrive/d/buildbot/worker/package_win64/build/src\task.c:198
start_task at /cygdrive/d/buildbot/worker/package_win64/build/src\task.c:717
error in running finalizer: MethodError(f=typeof(Base.promote_type)(), args=(Core.TypeofBottom(), Int64), world=0x0000000000006cd8)
error in running finalizer: MethodError(f=typeof(Base.promote_type)(), args=(Core.TypeofBottom(), Int64), world=0x0000000000006cd8)

julia> versioninfo()
Julia Version 1.5.2
Commit 539f3ce943 (2020-09-23 23:17 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Xeon(R) E-2276M  CPU @ 2.80GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, skylake)

[JeffBezanson]

I think this case should be handled with sealed functions (#31222), so adding this method would be disallowed.