fix(NODE-3397): report more helpful error with unsupported authMechanism in initial handshake (#2876)

Author: vkarpov15

lib/core/connection/connect.js

@@ -179,6 +179,9 @@ function prepareHandshakeDocument(authContext, callback) {
     }
 
     const authProvider = AUTH_PROVIDERS[credentials.mechanism];
+    if (authProvider == null) {
+      return callback(new MongoError(`No AuthProvider for ${credentials.mechanism} defined.`));
+    }
     authProvider.prepare(handshakeDoc, authContext, callback);
     return;
   }