Make target-specific errors private

[newpavlov]

The main reason for existence of the target-specific errors is nicer error descriptions. Users arguably should not match on those error constants since our inner implementation may change together with possible error codes (for example, like with IOS_SEC_RANDOM). Marking the error constants as private we get flexibility to change error codes and names.

Additionally, the internal_desc change reduces generated binary size since compiler no longer has to include irrelevant error descriptions. Ideally the target-specific error descriptions would be defined in the relevant modules, but I couldn't find a good way to do it, so, unfortunately, I had to partially duplicate backend cfgs in internal_desc.

[josephlr]

This looks good and removes confusing stuff from the public API. I was also trying to figure out ways to put the descriptions in the platform-specific files, but I also don't know how to do this.

I'll keep thinking about it, but you should merge this in the meantime.