Better error message for field storage changes

New patch incorporating the suggested changes and adding in testing.

diff --git a/core/modules/field/src/Entity/FieldStorageConfig.php b/core/modules/field/src/Entity/FieldStorageConfig.php
index 91c619087b..fcfa7caf20 100644
--- a/core/modules/field/src/Entity/FieldStorageConfig.php
+++ b/core/modules/field/src/Entity/FieldStorageConfig.php
@@ -367,10 +367,10 @@ protected function preSaveUpdated(EntityStorageInterface $storage) {
 
     // Some updates are always disallowed.
     if ($this->getType() != $this->original->getType()) {
-      throw new FieldException("Cannot change the field type for an existing field storage.");
+      throw new FieldException(sprintf('Cannot change the field type for an existing field storage. The field storage %s has the type %s.', $this->id(), $this->original->getType()));
     }
     if ($this->getTargetEntityTypeId() != $this->original->getTargetEntityTypeId()) {
-      throw new FieldException("Cannot change the entity type for an existing field storage.");
+      throw new FieldException(sprintf('Cannot change the entity type for an existing field storage. The field storage %s has the type %s.', $this->id(), $this->original->getTargetEntityTypeId()));
     }
 
     // See if any module forbids the update by throwing an exception. This
diff --git a/core/modules/field/tests/src/Kernel/FieldStorageCrudTest.php b/core/modules/field/tests/src/Kernel/FieldStorageCrudTest.php
index 98cc95581d..fe2514fd2c 100644
--- a/core/modules/field/tests/src/Kernel/FieldStorageCrudTest.php
+++ b/core/modules/field/tests/src/Kernel/FieldStorageCrudTest.php
@@ -383,6 +383,42 @@ public function testUpdateFieldType() {
     }
   }
 
+  /**
+   * Test changing a field storage type.
+   */
+  public function testUpdateEntityType() {
+    $field_storage = FieldStorageConfig::create([
+      'field_name' => 'field_type',
+      'entity_type' => 'entity_test',
+      'type' => 'decimal',
+    ]);
+    $field_storage->save();
+
+    $this->expectException(FieldException::class);
+    $this->expectExceptionMessage('Cannot change the field type for an existing field storage. The field storage entity_test.field_type has the type decimal.');
+
+    $field_storage->set('type', 'foobar');
+    $field_storage->save();
+  }
+
+  /**
+   * Test changing a field storage entity type.
+   */
+  public function testUpdateEntityTargetType() {
+    $field_storage = FieldStorageConfig::create([
+      'field_name' => 'field_type',
+      'entity_type' => 'entity_test',
+      'type' => 'decimal',
+    ]);
+    $field_storage->save();
+
+    $this->expectException(FieldException::class);
+    $this->expectExceptionMessage('Cannot change the entity type for an existing field storage. The field storage foobar.field_type has the type entity_test.');
+
+    $field_storage->set('entity_type', 'foobar');
+    $field_storage->save();
+  }