refactor: improve error message for MongoTimeoutErrors

We will now surface the first encountered error in the set of
known server descriptions as the primary error message for a
timeout error. The `reason` field for server selection errors
will now include the `TopologyDescription` for better traceability

NODE-2397

Author: mbroadst

lib/core/error.js

@@ -95,9 +95,14 @@ class MongoParseError extends MongoError {
  */
 class MongoTimeoutError extends MongoError {
   constructor(message, reason) {
-    super(message);
+    if (reason && reason.error) {
+      super(reason.error);
+    } else {
+      super(message);
+    }
+
     this.name = 'MongoTimeoutError';
-    if (reason != null) {
+    if (reason) {
       this.reason = reason;
     }
   }

lib/core/sdam/server_selection.js

@@ -261,7 +261,7 @@ function selectServers(topology, selector, timeout, start, callback) {
   if (duration >= timeout) {
     return callback(
       new MongoTimeoutError(`Server selection timed out after ${timeout} ms`),
-      topology.description.error
+      topology.description
     );
   }
 
@@ -308,7 +308,7 @@ function selectServers(topology, selector, timeout, start, callback) {
       callback(
         new MongoTimeoutError(
           `Server selection timed out after ${timeout} ms`,
-          topology.description.error
+          topology.description
         )
       );
     }, timeout - duration);

test/functional/core/server.test.js

@@ -1002,8 +1002,7 @@ describe('Server tests', function() {
           let err;
           try {
             expect(error).to.be.an.instanceOf(Error);
-            const errorMessage = error.reason ? error.reason.message : error.message;
-            expect(errorMessage).to.match(/but this version of the Node.js Driver requires/);
+            expect(error).to.match(/but this version of the Node.js Driver requires/);
           } catch (e) {
             err = e;
           }

test/functional/scram_sha_256.test.js

@@ -186,10 +186,7 @@ describe('SCRAM-SHA-256 auth', function() {
       return withClient(
         this.configuration.newClient({}, options),
         () => Promise.reject(new Error('This request should have failed to authenticate')),
-        err => {
-          const errMessage = err.reason ? err.reason.message : err;
-          expect(errMessage).to.match(/Authentication failed/);
-        }
+        err => expect(err).to.match(/Authentication failed/)
       );
     }
   });
@@ -223,10 +220,7 @@ describe('SCRAM-SHA-256 auth', function() {
         withClient(
           this.configuration.newClient({}, options),
           () => Promise.reject(new Error('This request should have failed to authenticate')),
-          err => {
-            const errMessage = err.reason ? err.reason.message : err;
-            expect(errMessage).to.match(/Authentication failed/);
-          }
+          err => expect(err).to.match(/Authentication failed/)
         );
 
       return Promise.all([getErrorMsg(noUsernameOptions), getErrorMsg(badPasswordOptions)]);

test/unit/sdam/server_selection/select_servers.test.js

@@ -35,7 +35,7 @@ describe('selectServers', function() {
     selectServers(topology, ReadPreference.primary, 500, process.hrtime(), err => {
       expect(err).to.exist;
       expect(err).to.match(/Server selection timed out/);
-      expect(err).to.not.have.property('reason');
+      expect(err).to.have.property('reason');
 
       done();
     });
@@ -60,7 +60,7 @@ describe('selectServers', function() {
       selectServers(topology, ReadPreference.primary, 1000, process.hrtime(), err => {
         expect(err).to.exist;
         expect(err).to.match(/Server selection timed out/);
-        expect(err).to.not.have.property('reason');
+        expect(err).to.have.property('reason');
 
         // expect a call to monitor for initial server creation, and another for the server selection
         expect(serverMonitor)