Attempting to write an invalid value can leave the cache in a broken state

[simonbyrne]

If I attempt to write an object that can't be represented by the TOML format, it can leave the cache in an invalid state:

julia> using Preferences

julia> set_preferences!(Preferences, "a" => 1) # set a key to initialize the cache

julia> set_preferences!(Preferences, "b" => v"1.0") # attempt to write an invalid object, throws an error
ERROR: type `VersionNumber` is not a valid TOML type, pass a conversion function to `TOML.print`
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:35
  [2] _print(f::Nothing, io::IOStream, a::Dict{String, Any}, ks::Vector{String}; indent::Int64, first_block::Bool, sorted::Bool, by::Function)
    @ TOML.Internals.Printer /Applications/Julia-1.8.app/Contents/Resources/julia/share/julia/stdlib/v1.8/TOML/src/print.jl:112
 ...

julia> set_preferences!(Preferences, "c" => 1) # now write a valid object
ERROR: type `VersionNumber` is not a valid TOML type, pass a conversion function to `TOML.print`
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:35
  [2] _print(f::Nothing, io::IOStream, a::Dict{String, Any}, ks::Vector{String}; indent::Int64, first_block::Bool, sorted::Bool, by::Function)
    @ TOML.Internals.Printer /Applications/Julia-1.8.app/Contents/Resources/julia/share/julia/stdlib/v1.8/TOML/src/print.jl:112
  [3] invokelatest(::Any, ::Any, ::Vararg{Any}; kwargs::Base.Pairs{Symbol, Any, NTuple{4, Symbol}, NamedTuple{(:indent, :first_block, :sorted, :by), Tuple{Int64, Bool, Bool, typeof(identity)}}})
    @ Base ./essentials.jl:731
  [4] _print(f::Nothing, io::IOStream, a::Dict{String, Any}, ks::Vector{String}; indent::Int64, first_block::Bool, sorted::Bool, by::Function)
    @ TOML.Internals.Printer /Applications/Julia-1.8.app/Contents/Resources/julia/share/julia/stdlib/v1.8/TOML/src/print.jl:147
  [5] #print#15
    @ /Applications/Julia-1.8.app/Contents/Resources/julia/share/julia/stdlib/v1.8/TOML/src/print.jl:170 [inlined]
  [6] (::Preferences.var"#4#5")(io::IOStream)
    @ Preferences ~/.julia/dev/Preferences/src/Preferences.jl:145
  [7] open(::Preferences.var"#4#5", ::String, ::Vararg{String}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Base ./io.jl:384
  [8] open
    @ ./io.jl:381 [inlined]
  [9] set_preferences!(target_toml::String, pkg_name::String, pairs::Pair{String, Int64}; force::Bool)
    @ Preferences ~/.julia/dev/Preferences/src/Preferences.jl:144
 [10] set_preferences!
    @ ~/.julia/dev/Preferences/src/Preferences.jl:115 [inlined]
 [11] set_preferences!(u::Base.UUID, prefs::Pair{String, Int64}; export_prefs::Bool, active_project_only::Bool, kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Preferences ~/.julia/dev/Preferences/src/Preferences.jl:283
 [12] set_preferences!
    @ ~/.julia/dev/Preferences/src/Preferences.jl:205 [inlined]
 [13] #set_preferences!#13
    @ ~/.julia/dev/Preferences/src/Preferences.jl:286 [inlined]
 [14] set_preferences!(m::Module, prefs::Pair{String, Int64})
    @ Preferences ~/.julia/dev/Preferences/src/Preferences.jl:285
 [15] top-level scope
    @ REPL[4]:1