Use markedAsFailure for exitCode and only set if undefined.

Respect markedAsFailure from your run and exit with an error. This
also only sets the error code if it's not set. That means you can
set the exitCode in your script and it will be respected.

bin/browsertime.js

@@ -149,11 +149,17 @@ async function run(urls, options) {
       const resultDirectory = relative(process.cwd(), storageManager.directory);
 
       // check for errors
-      for (let eachResult of result) {
-        for (let errors of eachResult.errors) {
-          if (errors.length > 0) {
+      // If we have set the exit code in scripts, respect that
+      if (process.exitCode === undefined) {
+        for (let eachResult of result) {
+          if (eachResult.markedAsFailure === 1) {
             process.exitCode = 1;
           }
+          for (let errors of eachResult.errors) {
+            if (errors.length > 0) {
+              process.exitCode = 1;
+            }
+          }
         }
       }
       log.info(`Wrote data to ${resultDirectory}`);