--- a/testing/marionette/addon.js
+++ b/testing/marionette/addon.js
@@ -82,32 +82,32 @@ async function installAddon(file) {
  */
 addon.install = async function(path, temporary = false) {
   let addon;
   let file;
 
   try {
     file = new FileUtils.File(path);
   } catch (e) {
-    throw new UnknownError(`${path} is not an absolute path.`);
+    throw new UnknownError(`Expected absolute path: ${e}`, e);
   }
 
   if (!file.exists()) {
-    throw new UnknownError(`Could not find add-on at '${path}'`);
+    throw new UnknownError(`No such file or directory: ${path}`);
   }
 
   try {
     if (temporary) {
       addon = await AddonManager.installTemporaryAddon(file);
     } else {
       addon = await installAddon(file);
     }
   } catch (e) {
     throw new UnknownError(
-        `Could not install add-on at '${path}': ${e.message}`);
+        `Could not install add-on: ${path}: ${e.message}`, e);
   }
 
   return addon.id;
 };
 
 /**
  * Uninstall a Firefox addon.
  *

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_addons.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_addons.py
@@ -91,14 +91,14 @@ class TestAddons(MarionetteTestCase):
         addon_path = os.path.join(here, "webextension-unsigned.xpi")
 
         with self.assertRaises(AddonInstallException):
             self.addons.install(addon_path)
 
     def test_install_nonexistent_addon(self):
         addon_path = os.path.join(here, "does-not-exist.xpi")
 
-        with self.assertRaisesRegexp(AddonInstallException, "Could not find add-on at"):
+        with self.assertRaises(AddonInstallException):
             self.addons.install(addon_path)
 
     def test_install_with_relative_path(self):
-        with self.assertRaisesRegexp(AddonInstallException, "is not an absolute path."):
+        with self.assertRaises(AddonInstallException):
             self.addons.install('webextension.xpi')