refactor: don't store errors directly on TopologyDescription

Propagating and storing an error on the `TopologyDescription` has
the unintended side-effect of never removing said error, thus
effectively poisoning the description in the event of failure
recovery.

Author: mbroadst

lib/core/sdam/topology_description.js

@@ -28,8 +28,7 @@ class TopologyDescription {
     maxSetVersion,
     maxElectionId,
     commonWireVersion,
-    options,
-    error
+    options
   ) {
     options = options || {};
 
@@ -47,7 +46,6 @@ class TopologyDescription {
     this.logicalSessionTimeoutMinutes = null;
     this.heartbeatFrequencyMS = options.heartbeatFrequencyMS || 0;
     this.localThresholdMS = options.localThresholdMS || 0;
-    this.error = error;
     this.commonWireVersion = commonWireVersion || null;
 
     // save this locally, but don't display when printing the instance out
@@ -133,7 +131,6 @@ class TopologyDescription {
     let maxSetVersion = this.maxSetVersion;
     let maxElectionId = this.maxElectionId;
     let commonWireVersion = this.commonWireVersion;
-    let error = serverDescription.error || this.error;
 
     const serverType = serverDescription.type;
     let serverDescriptions = new Map(this.servers);
@@ -159,8 +156,7 @@ class TopologyDescription {
         maxSetVersion,
         maxElectionId,
         commonWireVersion,
-        this.options,
-        error
+        this.options
       );
     }
 
@@ -241,11 +237,17 @@ class TopologyDescription {
       maxSetVersion,
       maxElectionId,
       commonWireVersion,
-      this.options,
-      error
+      this.options
     );
   }
 
+  get error() {
+    const descriptionsWithError = Array.from(this.servers.values()).filter(sd => sd.error);
+    if (descriptionsWithError.length > 0) {
+      return descriptionsWithError[0].error;
+    }
+  }
+
   /**
    * Determines if the topology description has any known servers
    */