Soft fail in CustomPropertiesBackend whenever storage not available

When a storage is not available, it will not fail the whole call any
more but still return a usable file list.

lib/private/connector/sabre/custompropertiesbackend.php

@@ -30,6 +30,7 @@ use Sabre\DAV\PropFind;
 use Sabre\DAV\PropPatch;
 use Sabre\DAV\Tree;
 use Sabre\DAV\Exception\NotFound;
+use Sabre\DAV\Exception\ServiceUnavailable;
 
 class CustomPropertiesBackend implements BackendInterface {
 
@@ -100,6 +101,9 @@ class CustomPropertiesBackend implements BackendInterface {
 			if (!($node instanceof Node)) {
 				return;
 			}
+		} catch (ServiceUnavailable $e) {
+			// might happen for unavailable mount points, skip
+			return;
 		} catch (NotFound $e) {
 			// in some rare (buggy) cases the node might not be found,
 			// we catch the exception to prevent breaking the whole list with a 404

tests/lib/connector/sabre/custompropertiesbackend.php

@@ -105,11 +105,16 @@ class CustomPropertiesBackend extends \Test\TestCase {
 	 * Test that propFind on a missing file soft fails
 	 */
 	public function testPropFindMissingFileSoftFail() {
-		$this->tree->expects($this->any())
+		$this->tree->expects($this->at(0))
 			->method('getNodeForPath')
 			->with('/dummypath')
 			->will($this->throwException(new \Sabre\DAV\Exception\NotFound()));
 
+		$this->tree->expects($this->at(1))
+			->method('getNodeForPath')
+			->with('/dummypath')
+			->will($this->throwException(new \Sabre\DAV\Exception\ServiceUnavailable()));
+
 		$propFind = new \Sabre\DAV\PropFind(
 			'/dummypath',
 			array(
@@ -125,6 +130,11 @@ class CustomPropertiesBackend extends \Test\TestCase {
 			$propFind
 		);
 
+		$this->plugin->propFind(
+			'/dummypath',
+			$propFind
+		);
+
 		// no exception, soft fail
 		$this->assertTrue(true);
 	}