Improve stacktrace of errors thrown from within a callback

[odow]

Closes #232

Now gives:

julia> using GLPK; import MathOptInterface as MOI

julia> model = GLPK.Optimizer()
A GLPK model

julia> x = MOI.add_variable(model)
MOI.VariableIndex(1)

julia> MOI.add_constraint(model, x, MOI.ZeroOne())
MathOptInterface.ConstraintIndex{MathOptInterface.VariableIndex, MathOptInterface.ZeroOne}(1)

julia> my_callback(args...) = error("FooBar")
my_callback (generic function with 1 method)

julia> MOI.set(model, GLPK.CallbackFunction(), my_callback)

julia> MOI.optimize!(model)
ERROR: FooBar
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:35
  [2] my_callback(args::GLPK.CallbackData)
    @ Main ./REPL[5]:1
  [3] (::GLPK.var"#17#18"{GLPK.Optimizer, typeof(my_callback)})(cb_data::GLPK.CallbackData)
    @ GLPK ~/.julia/dev/GLPK/src/MOI_wrapper/MOI_callbacks.jl:31
  [4] _internal_callback(tree::Ptr{Nothing}, info::Ptr{Nothing})
    @ GLPK ~/.julia/dev/GLPK/src/MOI_wrapper/MOI_wrapper.jl:232
  [5] glp_intopt
    @ ~/.julia/dev/GLPK/src/gen/libglpk_api.jl:342 [inlined]
  [6] _solve_mip_problem(model::GLPK.Optimizer)
    @ GLPK ~/.julia/dev/GLPK/src/MOI_wrapper/MOI_wrapper.jl:1399
  [7] optimize!(model::GLPK.Optimizer)
    @ GLPK ~/.julia/dev/GLPK/src/MOI_wrapper/MOI_wrapper.jl:1448

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.74%. Comparing base (596b3a8) to head (5911b02).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
+ Coverage   85.71%   85.74%   +0.02%     
==========================================
  Files           8        8              
  Lines        1911     1915       +4     
==========================================
+ Hits         1638     1642       +4     
  Misses        273      273              

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