--- a/testing/mochitest/server.js
+++ b/testing/mochitest/server.js
@@ -121,17 +121,17 @@ function runServer()
 {
   serverBasePath = __LOCATION__.parent;
   server = createMochitestServer(serverBasePath);
 
   //verify server address
   //if a.b.c.d or 'localhost'
   if (typeof(_SERVER_ADDR) != "undefined") {
     if (_SERVER_ADDR == "localhost") {
-      gServerAddress = _SERVER_ADDR;      
+      gServerAddress = _SERVER_ADDR;
     } else {
       var quads = _SERVER_ADDR.split('.');
       if (quads.length == 4) {
         var invalid = false;
         for (var i=0; i < 4; i++) {
           if (quads[i] < 0 || quads[i] > 255)
             invalid = true;
         }
@@ -167,25 +167,27 @@ function runServer()
 
   if (typeof(_PROFILE_PATH) == "undefined") {
     serverAlive.initWithFile(serverBasePath);
     serverAlive.append("mochitesttestingprofile");
   } else {
     serverAlive.initWithPath(_PROFILE_PATH);
   }
 
-  // If we're running outside of the test harness, there might
-  // not be a test profile directory present
+  // Create a file to inform the harness that the server is ready
   if (serverAlive.exists()) {
     serverAlive.append("server_alive.txt");
     foStream.init(serverAlive,
                   0x02 | 0x08 | 0x20, 436, 0); // write, create, truncate
     var data = "It's alive!";
     foStream.write(data, data.length);
     foStream.close();
+  } else {
+    throw new Error("Failed to create server_alive.txt because " + serverAlive.path +
+                    " could not be found.");
   }
 
   makeTags();
 
   //
   // The following is threading magic to spin an event loop -- this has to
   // happen manually in xpcshell for the server to actually work.
   //
@@ -299,17 +301,17 @@ function processLocations(server)
     {
       if (options.split(",").includes("primary"))
       {
         if (seenPrimary)
         {
           throw "Multiple primary locations in server-locations.txt, " +
                 "line " + lineno;
         }
-  
+
         server.identity.setPrimary(scheme, host, port);
         seenPrimary = true;
         continue;
       }
     }
 
     server.identity.add(scheme, host, port);
   }
@@ -446,17 +448,17 @@ function isTest(filename, pattern)
     return pattern.test(filename);
 
   // File name is a URL style path to a test file, make sure that we check for
   // tests that start with the appropriate prefix.
   var testPrefix = typeof(_TEST_PREFIX) == "string" ? _TEST_PREFIX : "test_";
   var testPattern = new RegExp("^" + testPrefix);
 
   var pathPieces = filename.split('/');
-    
+
   return testPattern.test(pathPieces[pathPieces.length - 1]) &&
          !filename.includes(".js") &&
          !filename.includes(".css") &&
          !/\^headers\^$/.test(filename);
 }
 
 /**
  * Transform nested hashtables of paths to nested HTML lists.
@@ -466,17 +468,17 @@ function linksToListItems(links)
   var response = "";
   var children = "";
   for (let link in links) {
     const value = links[link];
     var classVal = (!isTest(link) && !(value instanceof Object))
       ? "non-test invisible"
       : "test";
     if (value instanceof Object) {
-      children = UL({class: "testdir"}, linksToListItems(value)); 
+      children = UL({class: "testdir"}, linksToListItems(value));
     } else {
       children = "";
     }
 
     var bug_title = link.match(/test_bug\S+/);
     var bug_num = null;
     if (bug_title != null) {
         bug_num = bug_title[0].match(/\d+/);
@@ -751,10 +753,10 @@ function defaultDirHandler(metadata, res
   try {
     if (metadata.path.indexOf("/tests") != 0) {
       regularListing(metadata, response);
     } else {
       testListing(metadata, response);
     }
   } catch (ex) {
     response.write(ex);
-  }  
+  }
 }