topology: Make get_location() errors more verbose

Currently if topology.get_location() doesn't find an entry in its
collection(s) it throws standard out-of-range exception which's very
hard to debug.

Also, next patches will extend this method, the introduced here if
(_current_locations.contains()) makes this future change look nicer.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

locator/token_metadata.cc

@@ -1299,7 +1299,11 @@ bool topology::has_endpoint(inet_address ep) const
 }
 
 const endpoint_dc_rack& topology::get_location(const inet_address& ep) const {
-    return _current_locations.at(ep);
+    if (_current_locations.contains(ep)) {
+        return _current_locations.at(ep);
+    }
+
+    on_internal_error(tlogger, format("Node {} is not in topology", ep));
 }
 
 // FIXME -- both methods below should rather return data from the