Recommend log command line as error if fails

Recommend integrators implementing Secure scanner
to log command lines with the error log level
when these command lines fail.

doc/development/integrations/secure.md

@@ -260,6 +260,8 @@ When executing command lines, scanners should use the `debug` level to log the c
 For instance, the [bundler-audit](https://gitlab.com/gitlab-org/security-products/analyzers/bundler-audit) scanner
 uses the `debug` level to log the command line `bundle audit check --quiet`,
 and what `bundle audit` writes to the standard output.
+If the command line fails, then it should be logged with the `error` log level;
+this makes it possible to debug the problem without having to change the log level to `debug` and rerun the scanning job.
 
 #### common logutil package