Better error message for missing guesses

[AayushSabharwal]

Currently errors for missing guesses say "missing initial value" or "circular initial condition" (depending on where it originates). See:

ERROR: LoadError: Found symbolic value <<VALUE>> for variable <<VARIABLE>>. You may be missing an initial condition or have cyclic initial conditions. If this is intended, pass `symbolic_u0 = true`. In case the initial conditions are not cyclic but require more substitutions to resolve, increase `substitution_limit`. To report cycles in initial conditions of unknowns/parameters, pass `warn_cyclic_dependency = true`. If the cycles are still not reported, you may need to pass a larger value for `circular_dependency_max_cycle_length` or `circular_dependency_max_cycles`.


Stacktrace:
  [1] better_varmap_to_vars(varmap::Dict{Any, Any}, vars::Vector{SymbolicUtils.BasicSymbolic{Real}}; tofloat::Bool, use_union::Bool, container_type::Type, toterm::Function, promotetoconcrete::Nothing, check::Bool, allow_symbolic::Bool)
    @ ModelingToolkit ~/.julia/packages/ModelingToolkit/41wGH/src/systems/problem_utils.jl:340
  [2] process_SciMLProblem(constructor::Type, sys::ModelingToolkit.NonlinearSystem, u0map::Dict{Any, Any}, pmap::Dict{Any, Any}; build_initializeprob::Bool, implicit_dae::Bool, t::Nothing, guesses::Dict{Any, Any}, warn_initialize_determined::Bool, initialization_eqs::Vector{Any}, eval_expression::Bool, eval_module::Module, fully_determined::Nothing, check_initialization_units::Bool, tofloat::Bool, use_union::Bool, u0_constructor::typeof(identity), du0map::Nothing, check_length::Bool, symbolic_u0::Bool, warn_cyclic_dependency::Bool, circular_dependency_max_cycle_length::Int64, circular_dependency_max_cycles::Int64, substitution_limit::Int64, use_scc::Bool, kwargs::@Kwargs{})
    @ ModelingToolkit ~/.julia/packages/ModelingToolkit/41wGH/src/systems/problem_utils.jl:720

This error message should instead report a missing guess, since it is currently very misleading. The only way for a user to differentiate is via the stacktrace, which is bad for any number of reasons.