Allow Printing and Proxy service not to be present in cms.conf.

Both are required only in specific situation: when printing is enabled
and when there are external rankings, hence it makes sense to skip
them from the configuration if these features are not enabled.
Added logging explanation to two exceptions.

First is "wrong connection string", second is "service not specified
in conf". First still has several occurrence - we fixed only the case
in which the service accepts a contest id.

I introduced a new exception, ConfigError, which is supposed to be
used for critical configuration problems, and is catched only in the
scripts' main.