More helpful error messages for calling log/sqrt with negative real arguments

[sloede]

Currently, when log, sqrt, and friends are called with a negative real argument, the following error message is shown:

sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x))

While technically correct, it IMHO misses some clarity and does not convey to the user what the actual mistake was. I thus propose to change the error message to something like the following:

sqrt was called with a negative real argument but will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).

This should make it immediately clear to the regular user what happened.

I am proposing this PR since I noticed that many of our students who use our numerical simulation package do not know what to make of the original error, since they are not calling log(-2.0) etc. directly, but the call to log is hidden somewhere deep in the call stack. I would assume that this resembles the situation of many (most?) other users who will encounter this error.

cc @ChrisRackauckas

[sloede]

bump Is there something needed from my side to make this PR progress further?

[oscardssmith]

the test failures look real.

[sloede]

the test failures look real.

@oscardssmith My bad, that was an oversight. Now only i686-windows fail (which seems unrelated; the infamous "not empty" error) and powerpc64le/Linux (allowed to fail). Please let me know if I can/should do anything else.

[oscardssmith]

Thanks for the PR!