Improve debugging for ServiceUnavailable exceptions

I was getting a lot of these in my logs for no apparent reason, and file
uploads were failing:

	{"app":"webdav","message":"Sabre\\DAV\\Exception\\ServiceUnavailable: ","level":4,"time":"2015-01-06T15:33:39+00:00"}

In order to debug it, I had to add unique messages to all the places where
this exception was thrown, to identify which one it was, and that made the
logs much more useful:

	{"app":"webdav","message":"Sabre\\DAV\\Exception\\ServiceUnavailable: Encryption is disabled","level":4,"time":"2015-01-06T15:36:47+00:00"}

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/private/files/storage/dav.php b/lib/private/files/storage/dav.php
index 887ec1972b3..355148de37a 100644
--- a/lib/private/files/storage/dav.php
+++ b/lib/private/files/storage/dav.php
@@ -529,7 +529,7 @@ class DAV extends \OC\Files\Storage\Common {
 		} catch (Exception\NotFound $e) {
 			return false;
 		} catch (Exception $e) {
-			throw new StorageNotAvailableException();
+			throw new StorageNotAvailableException(get_class($e).": ".$e->getMessage());
 		}
 	}
 }