Add test in common/error module.
Add test in common/error module.

This is required for coverage when the common/error module is run with just the source files required to make it run, rather than all source files as we do now.

Likely something in the harness is providing coverage, but cover it explicitly so the coverage won't be lost if the harness changes.

pgmasters/backrest,pgbackrest/pgbackrest,pgbackrest/pgbackrest,pgmasters/backrest,pgbackrest/pgbackrest