Fix error logging when thrown errors are strings

src/workerfarm/errorUtils.js

@@ -1,22 +1,30 @@
 function errorToJson(error) {
-  let jsonError = {
-    message: error.message,
-    stack: error.stack,
-    name: error.name
-  };
-  // Add all custom codeFrame properties
-  Object.keys(error).forEach(key => {
-    jsonError[key] = error[key];
-  });
-  return jsonError;
+  if (typeof error === 'string') {
+    return {message: error};
+  }
+
+  if (error instanceof Error) {
+    let jsonError = {
+      message: error.message,
+      stack: error.stack,
+      name: error.name
+    };
+    // Add all custom codeFrame properties
+    Object.keys(error).forEach(key => {
+      jsonError[key] = error[key];
+    });
+    return jsonError;
+  }
 }
 
 function jsonToError(json) {
-  let error = new Error(json.message);
-  Object.keys(json).forEach(key => {
-    error[key] = json[key];
-  });
-  return error;
+  if (json) {
+    let error = new Error(json.message);
+    Object.keys(json).forEach(key => {
+      error[key] = json[key];
+    });
+    return error;
+  }
 }
 
 exports.errorToJson = errorToJson;