Output formatting typo in `rse/protocols/gfal.py`
 ##Motivation
----------
Previous PR #2555 introduced this bug related to a missing parentheses at the formatting of a debug output string here: https://github.com/rucio/rucio/blob/916987976a1f994a64e4ef3dce837b49141a8f1b/lib/rucio/rse/protocols/gfal.py#L363.


Modification
------------
Add the parentheses surrounding the formatting elements.


diff --git a/lib/rucio/rse/protocols/gfal.py b/lib/rucio/rse/protocols/gfal.py
index d5ce34c36..18fa80b99 100644
--- a/lib/rucio/rse/protocols/gfal.py
+++ b/lib/rucio/rse/protocols/gfal.py
@@ -360,7 +360,7 @@ class Default(protocol.RSEProtocol):
                 ret[checksum_name] = ctx.checksum(str(path), str(checksum_name.capitalize()))
                 verified = True
             except Exception as error:
-                message += 'Error while processing gfal checksum call (%s). Error: %s \\n' % checksum_name, str(error)
+                message += 'Error while processing gfal checksum call (%s). Error: %s \\n' % (checksum_name, str(error))
 
         if not verified:
             raise exception.RSEChecksumUnavailable(message)
['lib/rucio/rse/protocols/gfal.py']