Make sccache more prominently the source of error messages

[glandium]

We have an increase in the number of builds that fail on Firefox CI with

error: failed to execute compile
caused by: error reading compile response from server
caused by: Failed to read response header
caused by: failed to fill whole buffer

Note "sccache" doesn't appear once in the error message. And that's not the only kind of error message one can get during a build due to sccache, and currently, it requires prior knowledge to know that they come from sccache (or a grep in the sccache repo, which one shouldn't have to do)