generator: Introduce error code (errcode) concept.

generator: Introduce error code (errcode) concept.

There was a lot of repeated code to map return types (eg. RErr)
to error cases (eg. -1 or NULL).

This commit introduces an error code type and two functions to
map return types to error codes and error codes to strings.

Cherry picked from commit 8037da06feea097716ce700f38c0eac0d5411a7c
and rebased against stable 1.8 branch.