loader: Fix loading when CVSROOT/config file is missing

When using the rsync protocol the loader parses the CVSROOT/config file to look for a custom keyword definition.

However, it exists CVS repositories where such file is missing and thus loadings were failing as file existence was not checked prior opening it.

Fixes SWH-LOADER-CVS-3P

Jenkins job DLDCVS/gitlab-builds #9 succeeded .
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merged