ignoring the error message

Previously it returned the return value of Logging::Logger#warn, which
is true for loggers that log the warn level. However
Diaspora::Federation::Receive::receive_relayable checks the return value
for truthiness in order to decide whether to attempt to relay it,
resulting in a NoMethodError: undefined method `parent' for
true:TrueClass in Diaspora::Federation::Receive::relay_relayable

This change is cosmetic as the exception raised prevented any action
that shouldn't happen anyway, so there's no actual logic change.