bug 1494613: geckodriver: log reason for not being able to write profile r=whimboo

mozprofile gives us an error and we can forward it to the user so
we know what the underlying reason is for not being able to write
the Firefox profile.

Depends on D7075

Differential Revision: https://phabricator.services.mozilla.com/D7076

--HG--
extra : moz-landing-system : lando

testing/geckodriver/src/marionette.rs

@@ -221,8 +221,11 @@ impl MarionetteHandler {
         );
         prefs.insert("marionette.port", Pref::new(port));
 
-        prefs.write().map_err(|_| {
-            WebDriverError::new(ErrorStatus::UnknownError, "Unable to write Firefox profile")
+        prefs.write().map_err(|e| {
+            WebDriverError::new(
+                ErrorStatus::UnknownError,
+                format!("Unable to write Firefox profile: {}", e),
+            )
         })
     }
 }