better error message for improper delay conversion

[baggepinnen]

julia> delay(5)*((s+1))/((s+2)*(s+0.5))
ERROR: The transfer function is not proper and can not be converted to a DelayLtiSystem type. If you tried to form the system `exp(sL) * B / A` where `B / A` is proper, add parenthesis to make it `exp(sL) * (B / A)`.

Ref #830

[codecov[bot]]

Codecov Report

Merging #831 (10412ce) into master (604a9ab) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #831   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files           4        4           
  Lines         315      315           
=======================================
  Hits          306      306           
  Misses          9        9           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[JuliaControlBot]

This is an automated message. Plots were compared to references. 11/11 images have changed, see differences below. After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here. Difference	Reference Image	New Image
:x: 0.047
:heavy_check_mark: 0.011
:heavy_check_mark: 0.001
:heavy_check_mark: 0.001
:heavy_check_mark: 0.007
:heavy_check_mark: 0.003
:heavy_check_mark: 0.0
:warning: 0.028
:x: 0.06
:heavy_check_mark: 0.0
:warning: 0.017