Unclear error when loadDFG and IIF is not imported.

[Affie]

The error results in calling loadDFG and without import/using IIF/RoME as SolverParams are defined in IIF, find a way to make the error clearer.

ERROR: MethodError: Cannot `convert` an object of type Nothing to an object of type Int64
Stacktrace:
  [1] NoSolverParams(d::Nothing)
    @ DistributedFactorGraphs ~/.julia/dev/DistributedFactorGraphs/src/entities/AbstractDFG.jl:37
  [2] construct
    @ ~/.julia/packages/StructTypes/AK4aM/src/StructTypes.jl:912 [inlined]
  [3] read(::StructTypes.UnorderedStruct, buf::Base.CodeUnits{…}, pos::Int64, len::Int64, b::UInt8, ::Type{…}; kw::@Kwargs{})
    @ JSON3 ~/.julia/packages/JSON3/jSAdy/src/structs.jl:646
  [4] read(::StructTypes.UnorderedStruct, buf::Base.CodeUnits{…}, pos::Int64, len::Int64, b::UInt8, ::Type{…})
    @ JSON3 ~/.julia/packages/JSON3/jSAdy/src/structs.jl:567
  [5] (::JSON3.StructClosure{Base.CodeUnits{UInt8, String}, @NamedTuple{}})(i::Int64, nm::Symbol, TT::Type; kw::@Kwargs{})
    @ JSON3 ~/.julia/packages/JSON3/jSAdy/src/structs.jl:557
  [6] StructClosure
    @ ~/.julia/packages/JSON3/jSAdy/src/structs.jl:555 [inlined]
  [7] applyfield
    @ ~/.julia/packages/StructTypes/AK4aM/src/StructTypes.jl:872 [inlined]
  [8] read(::StructTypes.UnorderedStruct, buf::Base.CodeUnits{…}, pos::Int64, len::Int64, b::UInt8, ::Type{…}; kw::@Kwargs{})
    @ JSON3 ~/.julia/packages/JSON3/jSAdy/src/structs.jl:618
  [9] read(::StructTypes.UnorderedStruct, buf::Base.CodeUnits{…}, pos::Int64, len::Int64, b::UInt8, ::Type{…})
    @ JSON3 ~/.julia/packages/JSON3/jSAdy/src/structs.jl:567
 [10] #read#53
    @ ~/.julia/packages/JSON3/jSAdy/src/structs.jl:744 [inlined]
 [11] read
    @ ~/.julia/packages/JSON3/jSAdy/src/structs.jl:740 [inlined]
 [12] read(str::String, ::Type{DistributedFactorGraphs.GraphsDFGs.PackedGraphsDFG}; jsonlines::Bool, kw::@Kwargs{})
    @ JSON3 ~/.julia/packages/JSON3/jSAdy/src/structs.jl:41
 [13] read
    @ ~/.julia/packages/JSON3/jSAdy/src/structs.jl:33 [inlined]
 [14] loadDFG(file::String)
    @ DistributedFactorGraphs ~/.julia/dev/DistributedFactorGraphs/src/FileDFG/services/FileDFG.jl:247
 [15] top-level scope
    @ REPL[32]:1
Some type information was truncated. Use `show(err)` to see complete types.