[Reviewer: Ellie] Add SAS log on transport failure

In https://github.com/Metaswitch/sprout/issues/538 you flagged that there's no SAS log on transport failure. This probably fixes that - unfortunately, I don't see an easy way to test it in the case that originally caused the problem (a P-CSCF tearing down connections due to overload).

https://github.com/Metaswitch/clearwater-sas-resources/commit/5e8ff2d39ebef0a7b781f4f9ce3d177c56aa36be is the corresponding resource bundle change.
The comment above says we should be treating this as a 504, but this ends up as a 408