Try to log the SASL username on errors

With the patch:
simta[18527.1381783028]: Auth [67.194.39.165] 67-194-39-165.wireless.umnet.umich.edu: ezekielh: sasl_start_server: SASL(-13): authentication failure: Password verification failed

Without:
simta[20532.1381783738]: Auth [67.194.39.165] 67-194-39-165.wireless.umnet.umich.edu: (null): sasl_start_server: SASL(-13): authentication failure: Password verification failed

Unlike the success path we don't check the return value of sasl_getprop, but that should be okay because we're on the error path and are going to fail the connection regardless.