(Frosting) Add better error message for "Task cannot be used"

[patriksvensson]

[gep13]

@patriksvensson also here, can you provide some more information? Thanks

[mgnslndh]

I'm not sure if this is the error message being referred to but it now seems better than simply "Task cannot be used" and the code was committed after the issue was reported:

cake/src/Cake.Frosting/Internal/FrostingEngine.cs

Lines 106 to 110 in a1893a2

	if (!task.HasCompatibleContext(_context))
	{
	const string format = "Task cannot be used since the context isn't convertible to {0}.";
	_log.Warning(format, task.GetContextType().FullName);
	}

Perhaps this is no longer an issue?