Fix false positive result of passing a Function to derivative

[kapple19]

As reported in #1085, e.g.

@variables x
Symbolics.derivative(sqrt, x)

returns 0, when it should return the derivative of sqrt (which is not zero of course).

Also added a catching method to throw for non-Num second input when first input isa Function.

@ChrisRackauckas mentions in #1085 to add a catching method to report "derivative not supported for typeof(f)" which I couldn't understand, I'll need further explanation? I thought it made more sense to report lack of support for x isa Any catch.

He also mentions a check on if x is symbolic in any form, I'm also not sure what that means, i.e. what other forms than Num are supported? Complex{Num} and Integer{Num}? (Which errors on input to f anyway.)

Happy to make changes as desired.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.42%. Comparing base (47cfd60) to head (6fcede0).
Report is 58 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1256       +/-   ##
===========================================
+ Coverage    8.30%   80.42%   +72.11%     
===========================================
  Files          46       46               
  Lines        4549     4658      +109     
===========================================
+ Hits          378     3746     +3368     
+ Misses       4171      912     -3259     

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