MEDIUM: spoe: Forward SPOE context error to the SPOE applet

MEDIUM: spoe: Forward SPOE context error to the SPOE applet

Errors triggered by a SPOE filter intance, mainly the processing timeout, are
now forwarded to the SPOE applet. This way, an error can be reported to the SPOP
mux stream to abort it early.

Note that, for now, no abort reaon is set because the SPOP connection is not
closed. Only the SPOP stream is aborted. But thanks to this patch, the SPOE
applet can be released immediately, instead of waiting for the ACK frame or an
error on the mux side.

The related issue is #2502.