Consistently with changes done to Utility::Path, this enforces proper
error handling on user side. Originally I didn't want to do this and
instead wanted to have a special Array instance devoted for an error
state, but that still would allow the error state be errorneously
treated as a successful but empty array.
It's more useful if the Error class is directly referenced than saying
just "error output" -- so people can grab it, redirect it, etc. Also
drop the useless "does what it is expected to do on success" sentences
that add no value whatsoever.
I wanted to avoid including extra stuff with the Manager.hpp split, but
this would make it even worse than having Array and String included
unconditionally. Fortunately it's enough to simply not even have the
declaration.
There will be Flag::FlipY for images at some point, enabled by default
for compatibility with existing GL code, and so it makes sense to start
discouraging setFlags() as early as possible to avoid people resetting
the default by accident.
Also update the imageconverter, sceneconverter and shaderconverter utils
to use these instead of setFlags().