Handle faulty doctest keyword arguments better

[mortenpi]

Does two things:

1. If Meta.parse fails to parse the doctest keyword arguments (x-ref Doctest syntax error, how to find out where? #1556), it will gracefully report a doctest failure and move on (rather than making makedocs throw an unhelpful Meta.ParseError).
2. If the keyword argument parsing fails, these now get logged as doctesting errors, and will fail the build with strict = true. Currently we just print a warning and move on.

Will close #1556. @fingolfin, do you mind trying out this branch for your case?

[fingolfin]

I can try it tomorrow. For the record, for some reason there was an isolated semicolon after jldoctest, i.e.:

```jldoctest;

[fingolfin]

It works nicely!

...
┌ Warning: Unable to parse doctest keyword arguments in ~/Projekte/OSCAR/Oscar.jl/src/Groups/GAPGroups.jl:326-332
│ Use ```jldoctest name; key1 = value1, key2 = value2
│
│ ```jldoctest;
│ julia> cperm([1,2,3],4:7)
│ (1,2,3)(4,5,6,7)
│
│ julia> cperm([1,2],[2,3])
│ (1,3,2)
│ ```
│   parse_error = Base.Meta.ParseError("unexpected \",\"")
└ @ Documenter.DocTests ~/Projekte/Julia/packages/Documenter.jl/src/DocTests.jl:144
...

Great, thank you!

[mortenpi]

Thanks for testing! Added it as a test case.