elevated ZSTD_getErrorCode() to stable status

[Cyan4973]

answering #4183.

Also:

• updated manual, so that ZSTD_getErrorCode() is clearly mentioned in the paragraph dealing with handling of errors
• updated version number, since this is technically a change of officially stable API

[Cyan4973]

mmmh,
including zstd_errors.h from zstd.h
seems in conflict with our single file library build system...

edit : nope, hopefully, it's just the test of the single file library which is failing.

But it does illustrate the fact that, since zstd.h now includes zstd_errors.h after this patch,
both files, zstd.h and zstd_errors.h, are now required,
which could be surprising if the build has been created with the assumption that only zstd.h is enough.