A better error msg if no underlying solver chosen

[guimarqu]

This is the current message :

ERROR: MethodError: no method matching optimize!(::Coluna.Formulation{Coluna.DwMaster}, ::Coluna.NoOptimizer)
Closest candidates are:
  optimize!(::Coluna.Formulation) at /Users/gmarques/.julia11/dev/Coluna/src/formulation.jl:433
  optimize!(::Coluna.Formulation, ::Coluna.UserOptimizer) at /Users/gmarques/.julia11/dev/Coluna/src/optimizerwrappers.jl:20
  optimize!(::Coluna.Formulation, ::Coluna.MoiOptimizer) at /Users/gmarques/.julia11/dev/Coluna/src/optimizerwrappers.jl:53
Stacktrace:
 [1] optimize!(::Coluna.Formulation{Coluna.DwMaster}) at /Users/gmarques/.julia11/dev/Coluna/src/formulation.jl:435
 [2] macro expansion at /Users/gmarques/.julia11/packages/TimerOutputs/7zSea/src/TimerOutput.jl:216 [inlined]
 [3] macro expansion at /Users/gmarques/.julia11/dev/Coluna/src/algorithms/colgen.jl:241 [inlined]
 [4] macro expansion at ./util.jl:213 [inlined]
 [5] solve_restricted_master!(::Coluna.Formulation{Coluna.DwMaster}) at /Users/gmarques/.julia11/dev/Coluna/src/algorithms/colgen.jl:240

[guimarqu]

fix by PR #203