feat(kerberos): expose warning for kerberos mismatch versions

We were originally blindly catching errors for a kerberos version
mismatch, and silently failing. This exposes the error message
to users if it exists.

Fixes NODE-1523

lib/auth/getKerberos.js

@@ -0,0 +1,18 @@
+'use strict';
+
+const requireOptional = require('require_optional');
+
+let Kerberos = null;
+let MongoAuthProcess = null;
+
+try {
+  const kerberos = requireOptional('kerberos');
+  if (kerberos) {
+    Kerberos = kerberos.Kerberos;
+    MongoAuthProcess = kerberos.processes.MongoAuthProcess;
+  }
+} catch (err) {
+  console.warn(err.message);
+}
+
+module.exports = { Kerberos, MongoAuthProcess };

lib/auth/gssapi.js

@@ -1,9 +1,12 @@
 'use strict';
 
-var f = require('util').format,
-  require_optional = require('require_optional'),
-  Query = require('../connection/commands').Query,
-  MongoError = require('../error').MongoError;
+const f = require('util').format;
+const Query = require('../connection/commands').Query;
+const MongoError = require('../error').MongoError;
+
+// Kerberos class
+const Kerberos = require('./getKerberos').Kerberos;
+const MongoAuthProcess = require('./getKerberos').MongoAuthProcess;
 
 var AuthSession = function(db, username, password, options) {
   this.db = db;
@@ -20,17 +23,6 @@ AuthSession.prototype.equal = function(session) {
   );
 };
 
-// Kerberos class
-var Kerberos = null;
-var MongoAuthProcess = null;
-
-// Try to grab the Kerberos class
-try {
-  Kerberos = require_optional('kerberos').Kerberos;
-  // Authentication process for Mongo
-  MongoAuthProcess = require_optional('kerberos').processes.MongoAuthProcess;
-} catch (err) {} // eslint-disable-line
-
 /**
  * Creates a new GSSAPI authentication mechanism
  * @class

lib/auth/sspi.js

@@ -1,9 +1,12 @@
 'use strict';
 
-var f = require('util').format,
-  require_optional = require('require_optional'),
-  Query = require('../connection/commands').Query,
-  MongoError = require('../error').MongoError;
+const f = require('util').format;
+const Query = require('../connection/commands').Query;
+const MongoError = require('../error').MongoError;
+
+// Kerberos class
+const Kerberos = require('./getKerberos').Kerberos;
+const MongoAuthProcess = require('./getKerberos').MongoAuthProcess;
 
 var AuthSession = function(db, username, password, options) {
   this.db = db;
@@ -20,17 +23,6 @@ AuthSession.prototype.equal = function(session) {
   );
 };
 
-// Kerberos class
-var Kerberos = null;
-var MongoAuthProcess = null;
-
-// Try to grab the Kerberos class
-try {
-  Kerberos = require_optional('kerberos').Kerberos;
-  // Authentication process for Mongo
-  MongoAuthProcess = require_optional('kerberos').processes.MongoAuthProcess;
-} catch (err) {} // eslint-disable-line
-
 /**
  * Creates a new SSPI authentication mechanism
  * @class