Sorting should throw when a comparison is not defined

[jishnub]

This fixes the following behavior on master:

julia> sort(Fill(im,2))
2-element Fill{Complex{Bool}}, with entries equal to im

This doesn't make sense, as complex numbers can't be ordered. After this,

julia> sort(Fill(im,2))
ERROR: MethodError: no method matching isless(::Complex{Bool}, ::Complex{Bool})

which matches the Base behavior:

julia> sort(fill(im,2))
ERROR: MethodError: no method matching isless(::Complex{Bool}, ::Complex{Bool})

[codecov]

Codecov Report

Merging #274 (81d2618) into master (e3c8eb9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #274   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files           5        5           
  Lines         836      851   +15     
=======================================
+ Hits          834      849   +15     
  Misses          2        2           

Impacted Files	Coverage Δ
src/FillArrays.jl	99.46% <100.00%> (+0.02%)	⬆️
src/fillalgebra.jl	100.00% <100.00%> (ø)

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