Better error on JLL unavailability (Windows, etc)

[jackn11]

(v1.7) pkg> add oneAPI
    Updating registry at `C:\Users\jackn\.julia\registries\General.toml`
   Resolving package versions...
    Updating `D:\Storage\Programming\julia\.julia\environments\v1.7\Project.toml`
  [8f75cd03] + oneAPI v0.2.4
    Updating `D:\Storage\Programming\julia\.julia\environments\v1.7\Manifest.toml`
  [8f75cd03] + oneAPI v0.2.4
  [700fe977] + NEO_jll v22.25.23529+0
  [85f0d8ed] + SPIRV_LLVM_Translator_unified_jll v0.2.0+0
  [6ac6d60f] + SPIRV_Tools_jll v2022.1.0+0
  [09858cae] + gmmlib_jll v22.1.3+0
  [94295238] + libigc_jll v1.0.11378+0
  [f4bc562b] + oneAPI_Level_Zero_Headers_jll v1.4.0+0
  [13eca655] + oneAPI_Level_Zero_Loader_jll v1.8.1+1

julia> oneAPI.versioninfo()
ERROR: UndefVarError: oneAPI not defined
Stacktrace:
 [1] top-level scope
   @ REPL[3]:1

julia> using oneAPI
ERROR: InitError: UndefVarError: libze_loader not defined
Stacktrace:
 [1] unsafe_zeInit
   @ C:\Users\jackn\.julia\packages\oneAPI\mHp15\lib\level-zero\libze.jl:1092 [inlined]
 [2] __init__()
   @ oneAPI.oneL0 C:\Users\jackn\.julia\packages\oneAPI\mHp15\lib\level-zero\oneL0.jl:94
 [3] _include_from_serialized(path::String, depmods::Vector{Any})
   @ Base .\loading.jl:768
 [4] _require_search_from_serialized(pkg::Base.PkgId, sourcepath::String)
   @ Base .\loading.jl:854
 [5] _require(pkg::Base.PkgId)
   @ Base .\loading.jl:1097
 [6] require(uuidkey::Base.PkgId)
   @ Base .\loading.jl:1013
 [7] require(into::Module, mod::Symbol)
   @ Base .\loading.jl:997
during initialization of module oneL0

[maleadt]

Only Linux is supported, as mentioned in the README.