proxy: NAT test errors should be visible without -verbose

On the standalone proxy currently NAT test errors are only visible if -verbose is activated. Those errors might be important as they set the proxy to restricted. Do we want to make them more visible?

From IRC:

10:47 <Sky[m]> hey, I noticed that some of my proxies are occasionally get time outs to the nat check probe server and therefore deciding that they are restricted, which results in almost no connections and so on...
10:47 <Sky[m]> I only see that message on the proxy with debug logging, so i thought that this message here https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/blob/main/proxy/lib/snowflake.go?ref_type=heads#L870 could maybe be raised to error?