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]

Codecov Report

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

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##           master     #831   +/-   ##
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  Coverage   97.14%   97.14%           
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  Lines         315      315           
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  Hits          306      306           
  Misses          9        9           

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