change: make `-error-on-replace` more cooperating

[simonpasquier]

With this change, the proxy wouldn't necessary return an error if -error-on-replace is set and the incoming query has a label matcher overlapping with the enforced label.

For instance when the enforced label is namespace="foo", the following expressions aren't rejected anymore:

• up{namespace!="bar"}
• up{namespace=~"foo|bar"}
• up{namespace!~"bar"}

The following expressions are rejected as before:

• up{namespace=""}
• up{namespace="bar"}
• up{namespace=~"bar|fred"}
• up{namespace!~"foo"}