Bug 1838152 - Improve error message when sending session.new without capabilities. r=Sasha,webdriver-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D206966

remote/shared/webdriver/Capabilities.sys.mjs

@@ -1031,7 +1031,10 @@ export function validateCapabilities(capabilities) {
  */
 export function processCapabilities(params) {
   const { capabilities } = params;
-  lazy.assert.object(capabilities);
+  lazy.assert.object(
+    capabilities,
+    lazy.pprint`Expected "capabilities" to be an object, got ${capabilities}`
+  );
 
   let {
     alwaysMatch: requiredCapabilities = {},