CSHARP-3180: Use server error codes in place of error messages when possible.

Author: DmitryLukyanov

src/MongoDB.Driver.Core/Core/Operations/DropCollectionOperation.cs

@@ -13,7 +13,6 @@
 * limitations under the License.
 */
 
-using System;
 using System.Threading;
 using System.Threading.Tasks;
 using MongoDB.Bson;
@@ -158,7 +157,9 @@ private WriteCommandOperation<BsonDocument> CreateOperation(ICoreSessionHandle s
 
         private bool ShouldIgnoreException(MongoCommandException ex)
         {
-            return ex.ErrorMessage == "ns not found";
+            return
+                ex.Code == (int)ServerErrorCode.NamespaceNotFound ||
+                ex.ErrorMessage == "ns not found";
         }
     }
 }

src/MongoDB.Driver.Core/Core/Operations/DropIndexOperation.cs

@@ -200,7 +200,9 @@ public async Task<BsonDocument> ExecuteAsync(IWriteBinding binding, Cancellation
 
         private bool ShouldIgnoreException(MongoCommandException ex)
         {
-            return ex.ErrorMessage != null && ex.ErrorMessage.Contains("ns not found");
+            return
+                ex.Code == (int)ServerErrorCode.NamespaceNotFound ||
+                ex.ErrorMessage != null && ex.ErrorMessage.Contains("ns not found");
         }
     }
 }

src/MongoDB.Driver.Core/ServerErrorCode.cs

@@ -30,6 +30,8 @@ internal enum ServerErrorCode
         Interrupted = 11601,
         InterruptedAtShutdown = 11600,
         InterruptedDueToReplStateChange = 11602,
+        MaxTimeMSExpired = 50,
+        NamespaceNotFound = 26,
         NetworkTimeout = 89,
         NotMaster = 10107,
         NotMasterNoSlaveOk = 13435,
@@ -43,7 +45,6 @@ internal enum ServerErrorCode
         StaleShardVersion = 63,
         UnknownReplWriteConcern = 79,
         UnsatisfiableWriteConcern = 100,
-        WriteConcernFailed = 64,
-        MaxTimeMSExpired = 50
+        WriteConcernFailed = 64
     }
 }