MINOR: muxes: Add exit status for errors about not implemented features

The MUX_ES_NOTIMPL_ERR exit status is added to allow the multiplexers to
report errors about not implemented features. This will be used by the H1
mux to return 501-not-implemented errors.
diff --git a/src/log.c b/src/log.c
index a1de7e5..c24fdbd 100644
--- a/src/log.c
+++ b/src/log.c
@@ -2168,6 +2168,10 @@
 				status = 408;
 				s_flags = SF_ERR_CLITO | SF_FINST_R;
 				break;
+			case MUX_ES_NOTIMPL_ERR:
+				status = 501;
+				s_flags = SF_ERR_PRXCOND | SF_FINST_R;
+				break;
 			case MUX_ES_INTERNAL_ERR:
 				status = 500;
 				s_flags = SF_ERR_INTERNAL | SF_FINST_R;