Bug 1836146 - [devtools] Improve exception message in DAMP. r=devtools-reviewers,perftest-reviewers,nchevobbe,afinder

We weren't printing the exception message.
Error mesage and its related stacktrace were also off.

Allow exception to be a string.

Differential Revision: https://phabricator.services.mozilla.com/D179459

testing/talos/talos/tests/devtools/addon/content/damp.js

@@ -390,8 +390,9 @@ Damp.prototype = {
   },
 
   exception(e) {
-    this.error(e);
-    dump(e.stack + "\n");
+    const str =
+      "Exception: " + (e?.message || e) + "\n" + (e?.stack || "No stack");
+    this.error(str);
   },
 
   // Waits for any pending operations that may execute on Firefox startup and that