Throw a helpful error for begin...end in NL macros

[odow]

Closes #2448

I didn't realize that things like this worked in @objective:

julia> using JuMP

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, begin
           y = 2 * x
           z = y + 1
           2 * z + 2
       end)
4 x + 4

But we can't make this work in general for nonlinear expressions, so I've opted to just make it a more informative error:

julia> using JuMP

julia> model = Model();

julia> @variable(model, x);

julia> @NLexpression(model, begin sin(x) + 1 end)
ERROR: LoadError: `begin...end` blocks are not supported in nonlinear macros. The nonlinear expression must be a single statement.
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] _parse_NL_expr(::Symbol, ::Expr, ::Symbol, ::Int64, ::Symbol) at /Users/oscar/.julia/dev/JuMP/src/parse_nlp.jl:23
 [3] _process_NL_expr(::Symbol, ::Expr) at /Users/oscar/.julia/dev/JuMP/src/parse_nlp.jl:257
 [4] @NLexpression(::LineNumberNode, ::Module, ::Vararg{Any,N} where N) at /Users/oscar/.julia/dev/JuMP/src/macros.jl:1619
in expression starting at REPL[5]:1

[codecov]

Codecov Report

Merging #2463 (e6b6584) into master (c432f66) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2463      +/-   ##
==========================================
- Coverage   92.23%   92.19%   -0.05%     
==========================================
  Files          43       43              
  Lines        4652     4626      -26     
==========================================
- Hits         4291     4265      -26     
  Misses        361      361              

Impacted Files	Coverage Δ
src/parse_nlp.jl	95.10% <100.00%> (+4.65%)	⬆️
src/Containers/no_duplicate_dict.jl	92.85% <0.00%> (-7.15%)	⬇️
src/Containers/vectorized_product_iterator.jl	40.00% <0.00%> (-6.67%)	⬇️
src/Containers/SparseAxisArray.jl	80.26% <0.00%> (-3.95%)	⬇️
src/Containers/DenseAxisArray.jl	74.81% <0.00%> (-2.97%)	⬇️
src/Containers/generate_container.jl	81.48% <0.00%> (-0.67%)	⬇️
src/print.jl	90.71% <0.00%> (-0.31%)	⬇️
src/macros.jl	92.43% <0.00%> (+0.27%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c432f66...e6b6584. Read the comment docs.