Add extra logging for debugging 500 responses *not for dev branch*

[br1-msw]

@rkd-msw please sanity check that these are sensible checks. I don't think there are other sproutlets we can hit this on.

[rkday-pro]

Looks good as far as it goes. But:

rkd@ip-10-0-0-85:~/sprout$ grep -r PJSIP_SC_INTERNAL_SERVER_ERROR src
src/bgcfsproutlet.cpp:        pjsip_msg* rsp = create_response(req, PJSIP_SC_INTERNAL_SERVER_ERROR);
src/subscriptionsproutlet.cpp:    st_code = PJSIP_SC_INTERNAL_SERVER_ERROR;
src/hss_sip_mapping.cpp:        st_code = PJSIP_SC_INTERNAL_SERVER_ERROR;
src/thread_dispatcher.cpp:              reject_with_retry_header(rdata, PJSIP_SC_INTERNAL_SERVER_ERROR);
src/basicproxy.cpp:    reject_request(rdata, PJSIP_SC_INTERNAL_SERVER_ERROR);
src/basicproxy.cpp:    reject_request(rdata, PJSIP_SC_INTERNAL_SERVER_ERROR);
src/basicproxy.cpp:      status_code = PJSIP_SC_INTERNAL_SERVER_ERROR;
src/basicproxy.cpp:    pjsip_tsx_terminate(_tsx, PJSIP_SC_INTERNAL_SERVER_ERROR);//LCOV_EXCL_LINE
src/authenticationsproutlet.cpp:      rsp->line.status.code = PJSIP_SC_INTERNAL_SERVER_ERROR;
src/authenticationsproutlet.cpp:      rsp->line.status.reason = *pjsip_get_status_text(PJSIP_SC_INTERNAL_SERVER_ERROR);
src/registrarsproutlet.cpp:    st_code = PJSIP_SC_INTERNAL_SERVER_ERROR;
src/registrarsproutlet.cpp:    rsp->line.status.code = PJSIP_SC_INTERNAL_SERVER_ERROR;
src/mmtel.cpp:  case PJSIP_SC_INTERNAL_SERVER_ERROR:
src/bono.cpp:    reject_request(rdata, PJSIP_SC_INTERNAL_SERVER_ERROR);
src/bono.cpp:    reject_request(rdata, PJSIP_SC_INTERNAL_SERVER_ERROR);
src/bono.cpp:    reject_request(rdata, PJSIP_SC_INTERNAL_SERVER_ERROR);
src/bono.cpp:      return PJSIP_SC_INTERNAL_SERVER_ERROR; // LCOV_EXCL_LINE find_create_flow failure cases are all excluded already
src/bono.cpp:    send_response(PJSIP_SC_INTERNAL_SERVER_ERROR);
src/bono.cpp:    pjsip_tsx_terminate(_tsx, PJSIP_SC_INTERNAL_SERVER_ERROR);

Think the bgcfsproutlet.cpp, hss_sip_mapping.cpp and maybe basicproxy.cpp are worth looking at if you haven't already.

[br1-msw]

hss_sip_mapping/basicproxy already has a TRC_ERROR logs, and bgcfsproutlet TRC_WARNING so should already come out.