Improve the exception message when an entity form class does not exist

Problem/Motivation

EntityTypeManager::getFormObject() will throw an exception if either the operation or its associated form class does not exist. The exception message will be in the format

''The "%s" entity type did not specify a "%s" form class.''

This message should be adjusted to include information about both scenarios.

Steps to reproduce

Case-1: Operation does not exist.
Run the following code to see the exception.

\Drupal::entityTypeManager()->getFormObject('node', 'invalid_operation');

Case-2: Form class does not exist.
Edit "handlers" annotation of node entity (core/modules/node/src/Entity/Node.php) and change class name of any one of the form handlers. eg:

 *   handlers = {
 *     "form" = {
 *       "default" = "Drupal\node\NonExistentClass",
 *     }

In both cases, exception message would be in the format "The "%s" entity type did not specify a "%s" form class.".

Proposed resolution

Update the exception message.

When the operation not exist: "The "%s" entity type did not specify a "%s" form class.".
When the form class does not exist: "The "%s" form handler of the "%s" entity type specifies a non-existent class "%s"

Remaining tasks

• Create a patch/MR: Done ✅ 6406
• Add new test to validate exception message: Done ✅
• Review

User interface changes

Before patch

After patch

API changes

none

Data model changes

none

Release notes snippet

none