error_code: add new error code for external charges

For now, make sure we only support one currency (the one on the account).

See discussion https://github.com/killbill/killbill/commit/942e214d49e9c7ed89da76d972ee017d2d3ade58#commitcomment-6045547

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>