Improve error messages for failing connections and TLS upgrades
Further TLS upgrade error message improvements

 * Include connection string
 * Slightly more specific messages for handshake failure and other TLS alerts
Special case TLS alert "unknown CA", which is very common

Indicates a generic certificate verification problem, which we mention.
merge bug26270 into stable
Handle MQTT processor errors more gracefully
Avoid ugly error messages when the collector terminates on abrupt reader/processor termination

They are harmless but can really confuse the user.
merge bug26270 into stable (again)
merge stable into default