Export error codes from RAPI client module

Until now the error codes were not available from the RAPI client
module. A newly added unit test ensures all error codes are contained in
“ECODE_ALL”, as well as ensuring consistency between the RAPI client and
the authoritative “errors” module.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>