The error log is currently enabled only in the diagnostic mode. Ideally, we should configure whether the error log is enabled from cmake.

We should also determine what the defaults should be:

• The error log should be arguably always enabled in the diagnostic mode. Perhaps we can simply keep the current lines in error.h that enable it if HAVE_DIAGNOSTICS is enabled.
• Whether the error log is enabled in the release mode should be configurable from cmake, and it could be even enabled by default if the performance impact is negligible.