Added helpfull message for DomainError

[ivarne]

Some functions throw DomainError for Real arguments because of type
stability and that the correct answer is complex. This patch add a ugly
hack, inspecting the bactrace, to give newcommers a helpefull hint.

julia> sqrt(-1)
ERROR: DomainError
    sqrt will only return complex result if called width a complex argument. Eg: sqrt(complex(x))

julia> log(-1)
ERROR: DomainError
    log will only return complex result if called width a complex argument. Eg: log(complex(x))

See: #2144

I hate that this approach includes a list of functions (:log, :log2, :log10, :sqrt) that has to be updated and can not be extended by module writers, but from the discussion in #2144 it looks like something that might be accepted.

[JeffBezanson]

I'm ok with this approach since it is so non-invasive. It needs some tweaks though. It needs a break so it can only print the message once per backtrace, and it should also only print the message if the relevant function is the first non-empty entry in the backtrace. Lastly I think a call to show_backtrace is needed, since defining the 3-argument form of showerror overrides the normal way that call gets done.

[ivarne]

Thanks for the comments. I have now updated the PR. (Sorry for messing with the history, it felt like a good idea, but it doesn't look right)

I didn't add the Bessel functions because I only know them by name and did not want to do anything wrong. They also have more than one parameter so my message would be wrong. There is also a bunch of trigonometric functions that might throw DomainError, but I have never used them to get complex results, and don't know how that would work.

Does anybody want to add a function to the list of DomainError messages?