Use dedicated Exception classes for extension system

+++ b/core/lib/Drupal/Core/Extension/ExtensionList.php
@@ -227,7 +227,7 @@ public function exists($extension_name) {
+   * @throws \Drupal\Core\Extension\UnknownExtensionException

I think we should add them inside an Exception namespace. We are not consistent with that in core at all, but at least for me, having that additional namespace is helpful to keep things focused.

I went through the patch and the introduction of the additional exceptions totally make sense.

This is amazing work. Do you mind creating a change record documenting the new exception classes we have?

+++ b/core/lib/Drupal/Core/Extension/Exception/UninstalledExtensionException.php
@@ -0,0 +1,8 @@
+class UninstalledExtensionException extends \RuntimeException {}

+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -147,7 +149,7 @@ public function getDefault() {
     if (!isset($list[$name])) {
-      throw new \InvalidArgumentException("$name theme is not installed.");
+      throw new UninstalledExtensionException("$name theme is not installed.");

This is a bit of a BC break. I think we should continue extending from the InvalidArgumentException so existing catch statements continue to work.

+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -147,7 +149,7 @@ public function getDefault() {
   public function setDefault($name) {
     $list = $this->listInfo();
     if (!isset($list[$name])) {
-      throw new \InvalidArgumentException("$name theme is not installed.");
+      throw new UninstalledExtensionException("$name theme is not installed.");
     }

Isn't there a place for modules which could throw the same kind of exception?

I'm not sure how to answer #16, but this patch and the change record look great.

@borisson_
Yeah not sure what #16 was about.
I gave it another review and really liked it.

Testbot Snafu.

This was still a testbot fluke, back to rtbc.

Committed aae672c and pushed to 8.6.x. Thanks!

I have noticed two recent issues that could possibly be related or at least those more familiar with extension discovery may be able to help on:

• #3000293: User warning: The following profile is missing from the file system: xxx in drupal_get_filename() (line 276 of core/includes/bootstrap.inc).
• #3000677: Fatal error after upgrade to 8.6x [due to regression in extension system]