Make FromUrl and FromSource more friendly

[YegorStepanov]

1st commit

Change unhandled exception with message exception.
Add Unsupported checking directly to the methods, because It fails in .NET core (and they are public).

2nd commit

Hide it from IntelliSense cuz it not working and not recommended. Feel free to drop the commit if it isn't desired.

3rd commit

Change unhandled exception with message-exception for the case:

BenchmarkRunner.Run(typeof(GenericBenchmarkType<>));

#1908

[adamsitnik]

LGTM, thank you @YegorStepanov !

[YegorStepanov]

Ah, for some reason I thought that [EditorBrowsable(EditorBrowsableState.Never)] works in Rider, but no.

Issue: dotnet/roslyn#4434